Added mainAxisExtent to SliverGridDelegateWithFixedCrossAxisCount and SliverGridDelegateWithMaxCrossAxisExtent (#71376)

Fixes https://github.com/flutter/flutter/issues/71972

selectionEnabled was not taken into account in:

key movement handling in RenderEditable
mouse drag handling in TextSelectionGestureDetectorBuilder

This reverts commit ceca634e.

This reverts commit 358f9b0e.

This reverts commit b18f1b81.

This removes the last couple of doc references to the Window class in the framework, so that I can re-land the engine side change that removes the Window class (flutter/engine#22505).

This reverts usages of the binding's platformDispatcher to use window again temporarily, because there isn't a TestPlatformDispatcher yet, and so some tests were failing because they mocked the TestWindow to return certain things (locales) that were returning the real values instead of the test values.

Once I've created a TestPlatformDispatcher to allow fake data to be passed to it, we can go back to using the platformDispatcher in all of these places

Remove references to `Window`, and switch usages to `PlatformDispatcher` or `SingletonFlutterWindow` (#69617)

* Remove references to dart:ui.Window, and point usages to PlatformDispatcher or SingletonFlutterWindow, as appropriate

* remove new test platform dispatchers

* Amend documentation

* Fix crash when widgetspan does not produce a semantics node in render paragraph

* fix comments

* fixed corner case, added test, refactored semantics widget

* addressing comment

My yearly pass to fix spelling errors in the API dartdocs.