xwm: tell the shell the pid of the X clients

All the surfaces from all the X clients share the same wl_client so
wl_client_get_credentials can't be used to get the pid of the X
clients.
The shell may need to know the pid to be able to associate a surface
with e.g. a DBus service.

[Pekka: fixed trivial merge conflicts.]
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>

src/compositor.h

@@ -122,6 +122,7 @@ struct weston_shell_interface {
 				    int32_t x, int32_t y,
 				    int32_t width, int32_t height);
 	void (*set_maximized)(struct shell_surface *shsurf);
+	void (*set_pid)(struct shell_surface *shsurf, pid_t pid);
 };
 
 struct weston_animation {