Allow WM_QUIT to be modified or stopped e.g through an event filter

If the win32 eventdispatcher sees a WM_QUIT message in function: QEventDispatcherWin32::processEvents(), it directly triggers QCoreApplication::instance()->quit().QCoreApplication::quit() calls directly exit(0) which stops all further event processing. It would be helpful if the default action for WM_QUIT could be modified or stopped through the winEventFilter().

The reason for requesting this possibility is to be able to run some complex cleanup steps before the application quits. winEventFilter() is supposed to get all the events before Qt handles it (as per the docs) and it should be able to handle this event there, but with the current implementation it is not.