Fix QWidgetWindow sending duplicate drag-drop events

Fix QWidgetWindow sending duplicate drag-drop events

Unlike the other event handlers in QWidgetWindow, the drag-drop events are not followed by an
early return. This causes all drag-drop events to also be sent to the root widget of the window,
which is a bug. For example in the fridgemagnets example, where the target widget and
root widget are the same, the drop event is received twice.

Change-Id: I99e56ad8c48b3d31b0bd7c815cea8490edbf0af4
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@nokia.com>