Fix Xwayland check (and spelling)
With Xwayland 23.1 and above, the definitive way of checking for an Xwayland server is to check for the "XWAYLAND" extension.
Keep the old XRandR method for compatibility with older versions of Xwayland.
Also fix "Xwayland" spelling, it's Xwayland, not XWayland.
/cc @whot
see-also: xorg/xserver#1546