Adds support for discrete scroll events, such as those sent by a scroll wheel.

Includes the plumbing to convert, dispatch, and handle these events, as well as
Scrollable support for consuming them.

What KeepAlive does is different from the Offstage widget. Let's not confuse these two and be more speicific in the wording for KeepAlive.

* enable lint unnecessary_new

* fix tests

* fix tests

* fix tests

- remove an //ignore that is no longer needed
- fix some intrinsic methods that used the API incorrectly (shouldn't affect correctness but should make things a tiny bit more efficient)
- add some asserts to help track down bugs quicker
- update a TODO to point to the currently relevant bug
- fix some indenting
- improve the naming of some privates to improve readability

* Revert "manual rollback of of 18530 (#18531)"

This reverts commit 3f79f8cb.

* Fix the offset calculation

* Add a unit test

We have many hidden children because of iOS accessibility (see also #18410).

For example, in the tile screen of complex_layout app, there could be up to 20 children being painted without this PR. With this PR, only 12 get painted.

This should improve the performance of FL-53 by 40%.

`RenderSemanticsGestureHandler` is no longer a semantics boundary, which allows us to correctly mark disabled buttons as disabled without having their semantics size and semantics node id change unexpectedly.

Fixes https://github.com/flutter/flutter/issues/12589.
Fixes https://github.com/flutter/flutter/issues/11991.
See also https://github.com/flutter/flutter/issues/11993.

This change also required some refactoring to how we deal with `twoPaneSemantics` scrolling as it previously relied on `RenderSemanticsGestureHandler` being a semantics boundary. This should also make the underlying logic easier to understand.

In addition, the following minor changes are included in this PR:
* Removal of orphaned and unused `SemanticsConfiguration.isMergingDescendantsIntoOneNode`.
* Logic optimizations for `markNeedsSemanticsUpdate` .
* Fix for edge case where `MergeSemantics` failed to merge semantics.
* Use of emojis to better indicate leaf merging in the printed semantics tree.
* Better assert message for adding invisible child semantics nodes.
* Make some semantics tests robuster by not relying on creation order of SemanticsNode ids across test boundaries.

Fixes https://github.com/flutter/flutter/issues/13943.

* ensures that only semantics boundaries will be added to owner._nodesNeedingSemantics as expected by compiler.
* no longer throws assert if markNeedsSemanticsUpdate is called on non-semantic-boundary render object with a non-semantic-boundary parent.
* Fixes #13109.
* removes onlyLocalUpdates from markNeedsSemanticsUpdate as its no longer needed.

* enable lint prefer_const_constructors_in_immutables

* remove LabeledGlobalKey._

* Show accessibility frame while scrolling

* ++

* fix test

Diagnostics object is.

Fixes #11855

* a11y: handle left/right scrolls correctly

* fix keep alive test

* fix scaffold test

Refactor RenderObject.toStringDeep and Widget.toStringDeep code to use DiagnosticsNode.

* AutomaticKeepAlive

A Widget that listens for notifications from widgets that don't want to die.

* Automatically wrap SliverList and SliverGrid children in AutomaticKeepAlive widgets

* Fixes for review comments