QAccessibleMenuItem: Implement QAccessibleInterface::window().

The window is required to be able to determine the correct
scale factor for the screen when High DPI scaling is active.

Change-Id: I8f776684396d055754093f62e0d33bd7e23cb52b
Task-number: QTBUG-52943
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@theqtcompany.com>