WinRT: Handle window exposure when visibility changes

Expose events should be sent to application windows when the main view
becomes visible (again). This fixes a blank screen which may occur when
resuming an app from suspend.

Change-Id: I33dc00482ef17cdc954a71626a8ad3cd24361a64
Reviewed-by: Oliver Wolff <oliver.wolff@digia.com>
Windows Phone: Handle back-button press

When the back button is pressed, send an immediate key event to the
topmost window (or the application if there is no window). If it is
accepted, mark the native event as accepted and send a key release event.
This way, the application may call accept() on the KeyPress event for
Qt::Key_Back in order to create backstepping behaviors within the app.

This is in line with Android, which quits the app when the event is
ignored. On Windows Phone, the default behavior occurs when the event is
ignored, which is to back out of the application and leave it running
in the background.

Task-number: QTBUG-35951
Change-Id: I46d15478f441f73d3660370370689b2f9fa11f25
Reviewed-by: Oliver Wolff <oliver.wolff@digia.com>