Add the XWAYLAND extension

This extension exists to serve one purpose: reliably identifying
Xwayland. Previous attempts at doing so included querying root window
properties, output names or input device names. All these attempts are
somewhat unreliable. Instead, let's use an extension - where that
extension is present we have an Xwayland server.

Clients should never need to do anything but check whether the extension
exists through XQueryExtension/XListExtensions.

This extension provides a single QueryVersion request only, and
that is only to provide future compatibility if we ever need anything
other than "this extension exists" functionality.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Olivier Fourdan <ofourdan@redhat.com>