* 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#123634Closes#115874
* Remove references to dart:ui.Window, and point usages to PlatformDispatcher or SingletonFlutterWindow, as appropriate
* remove new test platform dispatchers
* Amend documentation
Move Flutter Gallery to dev/integration_tests/ as it is an older copy used only for testing. The current version of the Flutter Gallery now lives in https://github.com/flutter/gallery.