Add convenience function to QMenu to query tear-off position/geometry

Add functionality to QMenu to query the global position of its tear-off menu (QTornOffMenu in qmenu.cpp).

It is possible to programmatically show a tear-off at a specific location (using QMenu::showTearOffMenu), but once it has been created I don't believe it is possible to find out where the user may have moved it to. Added this feature would make it possible for applications to save and restore the positions of torn off menus. To do this only the tear-off's position would be needed (QPoint), but it might be useful in other situations to have access to the full geometry of the tear-off (QRect).