Allow contentItem of XrItem to have Window as well

Summary:
Enable the use of the Window QML type as the contentItem property of XrItem.

Description:
Currently, the Window QML type cannot be assigned directly to the contentItem property of XrItem (documentation reference).

In scenarios where users want to simulate a Qt application in a VR environment, they are required to modify the QML code by replacing Window with Item, even if it’s just a minor change.

To make it easier to showcase the seamless integration of existing Qt applications into VR environments, allowing the Window QML type to be assigned directly to contentItem would be highly beneficial. This improvement would eliminate the need for code modifications and strengthen the value proposition of using Qt for VR projects.