* Replace references to LinearProgressIndicator in CircularProgressIndicator example

* Add test

* fix copyright header

`DropdownButton`: Update `selectedItemBuilder` example to show case item alignment and update DropdownButton examples (#102748)

Implements a PlatformMenuBar widget and associated data structures for defining menu bars that use native APIs for rendering.

This PR includes:
A PlatformMenuBar class, which is a widget that menu bar data can be attached to for sending to the platform.
A PlatformMenuDelegate base, which is the type taken by a new WidgetsBinding.platformMenuDelegate.
An implementation of the above in DefaultPlatformMenuDelegate that talks to the built-in "flutter/menu" channel to talk to the built-in platform implementation. The delegate is so that a plugin could override with its own delegate and provide other platforms with native menu support using the same widgets to define the menus.
This is the framework part of the implementation. The engine part will be in flutter/engine#32080 (and flutter/engine#32358)

* Update theme_extension.1.dart

* empty commit

* update color names to be semantic

`Tooltip`: Add an example for `TooltipTriggerMode.manual` and add tests for existing `Tooltip` examples (#100554)

* first pass

* x

* x

* address feedback

* support multiple extensions

* add convenience function, Object ⇒ dynamic, lerping

* remove not-useful comment

* fix examples/api lower sdk constraint

* remove trailing spaces

* remove another pesky trailing space

* improve lerp

* address feedback

* hide map implementation from constructor and copyWith

* use iterableproperty

* Revert "hide map implementation from constructor and copyWith"

This reverts commit a6994af0046e3c90dbc9405cac628feb5b2d3031.

* slow down sample

* make theme extension params required

* add null check

* improve documentation

* fix hashCode and operator == overrides

* modify existing tests

* remove trailing spaces

* add all tests except lerping

* fix lerping bug

* add toString to themeExtension example

* add lerping test

* assume non-nullability in example

* address feedback

* update docs

* remove trailing space

* use Map.unmodifiable

[ReorderableListView] Update doc with example to make it clear `proxyDecorator` can overriden to customise an item when it is being dragged. (#91837)

This reverts commit 5fd259be.

This reverts commit 1c374c65.

This reverts commit cb378edc.

This does a cleanup of the examples, removing all of the "section" markers and extra comments that we don't need anymore now that the samples are no longer in the source code. It also removes the --template arguments from the {@tool dartpad} and {@tool sample} directives, since those are no longer used. It converts two examples that I discovered were still embedded into linked examples in the examples folder.

I didn't delete the templates from the snippets config folder yet, because there are still embedded samples in the dart:ui package from the engine that use them. Once dart:ui no longer uses the templates, they can be removed.

I bumped the version of the snippets package to pick up a change that allows removal of the --template argument.