Not all classes inheriting QAccessibleInterface implement
QAccessibleInterface::window(), so, go up the hierarchy to find
one that does.
The window is required to be able to determine the correct
scale factor for the screen when High DPI scaling is active.
Fixes multimonitor issues for QtWidgets.

Amends change d7a3b614

.

Task-number: QTBUG-52943
Change-Id: I81733765faee4d296bd6ea046bc2cecb018eb28d
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@theqtcompany.com>