Override cursor doesn't restore properly if QGuiApplication::restoreOverrideCursor() is called on top of widget that has a custom cursor defined.

The attached application can be used to demonstrate the issue. Both buttons have custom cursors defined. Upper button sets override cursor and lower button restores the most recently set override cursor. Once the last override cursor is restored, the cursor should change to the custom cursor of the lower button, but it will instead be the default cursor.

This happens because QtGui doesn't have dependency to QtWidgets, so QGuiApplication can't check if widget wants non-default cursor restored. A possible solution is to make QWindow have a custom cursor similarly to QWidget, and make QGuiApplication check and use that. Most of the generic cursor logic currently in QtWidgets could then probably be moved to QtGui side and benefit non-widgets applications as well.