wayland: Implement xwayland-shell-v1

As explained in [1], WL_SURFACE_ID is racy because wayland aggressively
reuses object ids. The xwayland-shell-v1 protocol intends to fix that by
two things:

* associating a serial number with each X11 window. This is to avoid
  potential XID reuse
* referring to the wayland surface by the wl_surface rather than
  specifying an object id

Unfortunately, we will have to maintain both legacy WL_SURFACE_ID and
WL_SURFACE_SERIAL for quiet some time until most instances of Xwayland
support the xwayland-shell-v1 protocol [2].

[1] https://gitlab.freedesktop.org/xorg/xserver/-/issues/1157
[2] https://gitlab.freedesktop.org/wayland/wayland-protocols/-/merge_requests/163