Don't propagate unsolicited events from QWidgetWindow to QWidget

Show and Hide events were not propagated before this change.
Add Timer, DynamicPropertyChange, ChildAdded, and ChildRemoved,
because they are supposed to be delivered to one QObject.

Also don't propagate these events if WA_DontShowOnScreen is set.

Change-Id: I134bf3909d46141e4d3e39f41983f493a4f35478
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>