qt: x11: better detection for xwayland and recommend native wayland over xwayland

I think it is now safe to warn the user, as wayland video integration has been available for a while and seems to be working well. We have also started supporting fractional scaling. Recent Qt also seems to have good support for Wayland.

Wayland has its own problems, such as always on top setting not working, but it is still better than X11 where v-sync is not supported and rendering is not threaded.

Request review @chub.