Windows App: Fixed minimize bug and replace polling with notifications

Please see: http://crbug.com/290845

Bug fix:
On Windows, when the original window is minimized via the context menu, the copycat window is not minimized. To fix the bug, the app needs to listen to AppWindow.onMinimized event and minimize both windows when this fires. 

Improvement:
Instead of polling, the app now listens to AppWindow.onBoundsChanged to watch for changes in the original window.  This demonstrates more of what is possible with the app window API.