Show the windowing environment on non MS-Windows/Apple systems, since X11/WAYLAND are selected startup there was no convenient way for users to know which back-end was being used. Include the windowing environment in the About splash & system-info.txt since it will be useful for handling bug reports. This commit adds a private API call not intended for general use as I would like to be able to remove this later and it's only needed in the specific case of testing if Blender is using WAYLAND or X11 (which maybe be used via XWayland). Python scripts can already inspect the system to check which windowing environment used, the API call is mainly useful for troubleshooting.