Fix QOleDropTarget object leak

1) call Release() to nativeExtra->dropTraget in
QWidgetPrivate::unregisterOleDnd() to prevent leak

2) call RevokeDragDrop() in QWidgetPrivate::setParent_sys() when the old
window handle is to be destroyed and the current widget has already
registered an ole drop target object to prevent leak

Change-Id: I767f4e0ea3e6cb5f14d72ba88e1f8f1bc8c0dc3c
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>