Expose xscreen in QScreen class

The QScreen class exposes manufacturer, serialNumber etc but it doesn't expose the xscreen that the QScreen is on (e.g. ":0.0" vs ":0.1"). In my use case I would like KDE Plasma to be able to filter QScreens that don't belong to the current DISPLAY environment variable.

This enhancement is to add a "getDisplay" or "getXScreen" method to QScreen which returns either an integer, or the display descriptor for the DISPLAY for the QScreen.