xwayland: add a more reliable way to detect Xwayland

Some X11 clients (e.g. Mesa) need to check whether the X11 server
is Xwayland. Add a root window property indicating this. Should be
more reliable than checking RandR output names, especially if we
want to mirror Wayland wl_output.name in the future.

Signed-off-by: Simon Ser <contact@emersion.fr>
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