We must do something when requestUpdate() is called on a QWidgetWindow.
The semantics of UpdateRequest for QWindow and QWidget are unfortunately
different: for widgets an UpdateRequest means "sync the backing store".
For QWindow it also involves marking as dirty.

Change-Id: Idf40b3fc0873652dc081edeb12c96b3007a126ef
Reviewed-by: Morten Johan Sørvig <morten.sorvig@theqtcompany.com>
Reviewed-by: Jørgen Lind <jorgen.lind@theqtcompany.com>