Implement framework interface for the dialog window archetype (flutter#176202)

## What's new?
- 🍏 Added the concept of dialogs to the windowing API. New
symbols:
   - `DialogWindowControllerDelegate`
   - `DialogWindowController`
   - `WindowingOwner.createDialogWindowController`
   - `DialogWindow`
   - `WindowControllerCommon`
- 🥘 Introduced the `WindowControllerCommon` mixin to
share common windowing functionality between controller types
- I have NOT yet implemented dialogs. This pull request ONLY adds the
idea to the API. Win32 implementation to follow :)

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

Author: mattkae

examples/multiple_windows/lib/app/main_window.dart

@@ -96,12 +96,14 @@ class _WindowsTable extends StatelessWidget {
         context: context,
         controller: regular,
       ),
+      DialogWindowController() => throw UnimplementedError(),
     };
   }
 
   static String _getWindowTypeName(BaseWindowController controller) {
     return switch (controller) {
       RegularWindowController() => 'Regular',
+      DialogWindowController() => 'Dialog',
     };
   }
 

examples/multiple_windows/lib/app/window_content.dart

@@ -33,6 +33,7 @@ class WindowContent extends StatelessWidget {
         controller: regular,
         child: MaterialApp(home: RegularWindowContent(window: regular)),
       ),
+      DialogWindowController() => throw UnimplementedError(),
     };
   }
 }