Windows: Use DND effect chosen in DragEnter/Move for Drop.

The value of pdwEffect passed to IOleDropTarget::Drop() is always
the one with which the drag was initiated.

Task-number: QTBUG-43466
Change-Id: I045fef634b55d4f113b393aa0ad4aa15d37db372
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>