xwayland: Implement xwayland_shell_v1

Implements the xwayland_shell protocol which makes the surface association happen via a shared serial, rather than sharing a wl_surface resource ID across an X atom.

This solves a race that can happen if the wl_surface associated with a WL_SURFACE_ID for a window was destroyed before the update of the atom was processed by the compositor and another surface (or other object) had taken its id due to recycling.

Closes: #1157 (closed)