When a QML menu has `popupType: Popup.Window`, checkable menu items don't work.

If you put `onCheckedChanged` on the menu item, it seems like when the user chooses the item, the check gets toggled twice, so it seems like it never gets checked at all.

See the attached files for a simple test case.

We really need to use window-type menus (not item or native type), so this is a big problem for us. If there's any kind of workaround I'd be very happy to hear about it.