Fix crash when deleting top level windows embedded into QGraphicsProxyWidget.

Clear proxyWidget pointer in slot
QGraphicsProxyWidgetPrivate::_q_removeWidgetSlot().

Task-number: QTBUG-29684
Task-number: QTBUG-33213
Change-Id: Ibdbd52aa810ca908e3d98daa00954345e7a93a48
Reviewed-by: Andreas Aardal Hanssen <andreas@hanssen.name>
(cherry picked from qtbase/ff2dbe609ef4482d66d1ecd135b4f53f6aff7e60)