Add Game Dashboard button widget as a sibling window.

* GamedashboardContext represents a dashboard instance for a specific
  game window.
* Move IsGameWindow() from `chromeos/ui/wm/window_util.h` to
  GameDashboardController

Bug: b:273640981
Test: Ran GameDashboard* ash_unittests
Change-Id: I9c18f878479fc7719be2912f197d8b5211cfc149
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4550045


Commit-Queue: Prameet Shah <phshah@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Auto-Submit: Prameet Shah <phshah@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1149429}