xwayland/xwm: introduce wlr_xwayland_surface_try_from_wlr_surface()
This new function replaces wlr_surface_is_xwayland_surface() and wlr_xwayland_surface_from_wlr_surface().
This new function replaces wlr_surface_is_xwayland_surface() and wlr_xwayland_surface_from_wlr_surface().