* Pulled `FinderBase` out of `Finder`
  * `FinderBase` can be used for any object, not just elements
  * Terminology was updated to be more "find" related
* Re-implemented `Finder` using `FinderBase<Element>`
  * Backwards compatibility maintained with `_LegacyFinderMixin`
* Introduced base classes for SemanticsNode finders
* Introduced basic SemanticsNode finders through `find.semantics`
* Updated some relevant matchers to make use of the more generic `FinderBase`

Closes #123634
Closes #115874

Fixes https://github.com/flutter/flutter/issues/132274.

This change enables Flutter to generate multiple Scenes to be rendered into separate FlutterViews from a single widget tree. Each Scene is described by a separate render tree, which are all associated with the single widget tree.

This PR implements the framework-side mechanisms to describe the content to be rendered into multiple views. Separate engine-side changes are necessary to provide these views to the framework and to draw the framework-generated Scene into them.

## Summary of changes

The details of this change are described in [flutter.dev/go/multiple-views](https://flutter.dev/go/multiple-views). Below is a high-level summary organized by layers.

### Rendering layer changes

* The `RendererBinding` no longer owns a single `renderView`. In fact, it doesn't OWN any `RenderView`s at all anymore. Instead, it offers an API (`addRenderView`/`removeRenderView`) to add and remove `RenderView`s that then will be MANAGED by the binding. The `RenderView` itself is now owned by a higher-level abstraction (e.g. the `RawView` Element of the widgets layer, see below), who is also in charge of adding it to the binding. When added, the binding will interact with the `RenderView` to produce a frame (e.g. by calling `compositeFrame` on it) and to perform hit tests for incoming pointer events. Multiple `RenderView`s can be added to the binding (typically one per `FlutterView`) to produce multiple Scenes.
* Instead of owning a single `pipelineOwner`, the `RendererBinding` now owns the root of the `PipelineOwner` tree (exposed as `rootPipelineOwner` on the binding). Each `PipelineOwner` in that tree (except for the root) typically manages its own render tree typically rooted in one of the `RenderView`s mentioned in the previous bullet. During frame production, the binding will instruct each `PipelineOwner` of that tree to flush layout, paint, semantics etc. A higher-level abstraction (e.g. the widgets layer, see below) is in charge of adding `PipelineOwner`s to this tree.
* Backwards compatibility: The old `renderView` and `pipelineOwner` properties of the `RendererBinding` are retained, but marked as deprecated. Care has been taken to keep their original behavior for the deprecation period, i.e. if you just call `runApp`, the render tree bootstrapped by this call is rooted in the deprecated `RendererBinding.renderView` and managed by the deprecated `RendererBinding.pipelineOwner`.

### Widgets layer changes

* The `WidgetsBinding` no longer attaches the widget tree to an existing render tree. Instead, it bootstraps a stand-alone widget tree that is not backed by a render tree. For this, `RenderObjectToWidgetAdapter` has been replaced by `RootWidget`.
* Multiple render trees can be bootstrapped and attached to the widget tree with the help of the `View` widget, which internally is backed by a `RawView` widget. Configured with a `FlutterView` to render into, the `RawView` creates a new `PipelineOwner` and a new `RenderView` for the new render tree. It adds the new `RenderView` to the `RendererBinding` and its `PipelineOwner` to the pipeline owner tree.
* The `View` widget can only appear in certain well-defined locations in the widget tree since it bootstraps a new render tree and does not insert a `RenderObject` into an ancestor. However, almost all Elements expect that their children insert `RenderObject`s, otherwise they will not function properly. To produce a good error message when the `View` widget is used in an illegal location, the `debugMustInsertRenderObjectIntoSlot` method has been added to Element, where a child can ask whether a given slot must insert a RenderObject into its ancestor or not. In practice, the `View` widget can be used as a child of the `RootWidget`, inside the `view` slot of the `ViewAnchor` (see below) and inside a `ViewCollection` (see below). In those locations, the `View` widget may be wrapped in other non-RenderObjectWidgets (e.g. InheritedWidgets).
* The new `ViewAnchor` can be used to create a side-view inside a parent `View`. The `child` of the `ViewAnchor` widget renders into the parent `View` as usual, but the `view` slot can take on another `View` widget, which has access to all inherited widgets above the `ViewAnchor`. Metaphorically speaking, the view is anchored to the location of the `ViewAnchor` in the widget tree.
* The new `ViewCollection` widget allows for multiple sibling views as it takes a list of `View`s as children. It can be used in all the places that accept a `View` widget.

## Google3

As of July 5, 2023 this change passed a TAP global presubmit (TGP) in google3: tap/OCL:544707016:BASE:545809771:1688597935864:e43dd651

## Note to reviewers

This change is big (sorry). I suggest focusing the initial review on the changes inside of `packages/flutter` first. The majority of the changes describe above are implemented in (listed in suggested review order):

* `rendering/binding.dart`
* `widgets/binding.dart`
* `widgets/view.dart`
* `widgets/framework.dart`

All other changes included in the PR are basically the fallout of what's implemented in those files. Also note that a lot of the lines added in this PR are documentation and tests.

I am also very happy to walk reviewers through the code in person or via video call, if that is helpful.

I appreciate any feedback.

## Feedback to address before submitting ("TODO")

It's time to say good bye to an old friend. 
It has outlived its usefulness.
Farewell, AbstractNode! 🫡

Follow-up to https://github.com/flutter/engine/pull/42493.

Make tester.startGesture less async, for better stack traces

Make Element tree root generic

Remove 1745 decorative breaks

Refactors accessibility guidelines to remove the single window assumption

Reland (3): Removes single window assumptions from `flutter_test`

Remove key simulation obsolete comments

* Revert "Remove references to BindingBase.window (#122119)"

This reverts commit c7681f00.

* Revert "Remove another reference to BindingBase.window (#122341)"

This reverts commit 6ec44450.

* Revert "Reland (2): Removes single window assumptions from `flutter_test` (#122233)"

This reverts commit eb3d317e.

* Revert "Remove single view assumption from TestViewConfiguration (#122352)"

This reverts commit 927289fb.

* Revert "Updates `flutter/test/cupertino` to no longer use `TestWindow` (#122325)"

This reverts commit 67e17e45.

* Revert "Updates `flutter/test/gestures` to no longer reference `TestWindow` (#122327)"

This reverts commit c2a5111c.

* Revert "Updates `flutter/test/rendering` to no longer use `TestWindow` (#122347)"

This reverts commit 28b65e08.

* Revert "Updates `flutter_localizations/test` to stop using `TestWindow` (#122321)"

This reverts commit 01367d52.

Reland (2): Removes single window assumptions from `flutter_test`

Revert "Reland: Removes single window assumptions from `flutter_test`"

Reland: Removes single window assumptions from `flutter_test`

This reverts commit f2dd19d5.

Removes single window assumptions from `flutter_test`

Move semantic-related bindings to SemanticsBinding

* Fix typos

* lowercase animated & opacity

* Undo typo fix

---------
Co-authored-by: Michael Goderbauer <goderbauer@google.com>

* Documentation for drag/fling offset

* Fix typo

* Fix typo 2

* Fix the docs_test

* Fix the grammar

* Use `curly_braces_in_flow_control_structures` for `packages/flutter_driver`

* Use `curly_braces_in_flow_control_structures` for `packages/flutter_goldens`

* Use `curly_braces_in_flow_control_structures` for `packages/flutter_goldens_client`

* Use `curly_braces_in_flow_control_structures` for `packages/flutter_localizations`

* Use `curly_braces_in_flow_control_structures` for `packages/flutter_test`

* Use `curly_braces_in_flow_control_structures` for `packages/flutter_web_plugins`

* fix comments

* Use `curly_braces_in_flow_control_structures` for `packages/integration_test`

* fix indentation

* Fix position of CupertinoContextMenu within Transform.scale (#97896)

* Fix after rebase

This reverts commit 6ddb99e9.

This updates the examples for PhysicalKeyboardKey and LogicalKeyboardKey to use Focus widgets that handle the keys instead of using RawKeyboardListener, since that usually leads people down the wrong path. Updated the See Also and added tests as well. Also exposed the `physicalKey` attribute for `tester.sendKeyEvent`.

This reverts commit 5fd259be.