wayland: Introduce XWayland surface role

Being a "XWayland window" should be considered equivalent to a role,
even though it is not part of any protocol anywhere. The commit doesn't
have any functional difference, but just makes it clear that an
wl_surface managed by XWayland have the same type of special casing as
surface roles as defined by the Wayland protocol.

As the semantics are more explicit given the role is defined, a comment
explaining why the semantics need to be how they are was added.

https://bugzilla.gnome.org/show_bug.cgi?id=744932