Allow changing bitmap while drag/drop operation

According to the documentation, the pixmap that is set by
QDrag::setPixmap() cannot be changed anymore after the drag operation has been started.

Drag target might want to modify this pixmap, or at least
switch off the pixmap, while the drag is moving above this special target widget.

Requesting that the restriction of the "fixed pixmap" should
be overcome in QTs future releases.

Propose an Interface, like:

QDragMoveEvent::setOverridePixmap(bool bUseOverridePixmap, const QPixmap & pixmap);
// pixmap can also be empty, which makes the pixmap invisible

and the QDragManager should internally restore the original pimap, when the dragtarget
get's the QDragLeave event.