Fix closing a QMenu by pressing Alt.

In QMenuPrivate::hideMenu(), delay clearing of 'causedPopup.widget'
to after the QMenu::close() call, so that it is still accessible
in QMenu::hideEvent() which calls QMenuBarPrivate::setCurrentAction(0)
if the caused widget is a QMenuBar.

Task-number: QTBUG-47377
Task-number: QTBUG-49592
Change-Id: Idbda48e918dae799afea84068a60d7383d7b4971
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>