ShellApp: Use stable sequence for id, not pointer address

As danw points out,

  "It's unique during the lifetime of the window, but reasonably likely to be
  reused by another window after this one is destroyed. Using
  meta_window_get_stable_sequence() might be better."

https://bugzilla.gnome.org/show_bug.cgi?id=648149

src/shell-app.c

@@ -777,11 +777,7 @@ _shell_app_new_for_window (MetaWindow      *window)
 
   app = g_object_new (SHELL_TYPE_APP, NULL);
 
-  /* For windows, its id is simply its pointer address as a string.
-   * There are various other alternatives, but the address is unique
-   * and unchanging, which is pretty much the best we can do.
-   */
-  app->window_id_string = g_strdup_printf ("window:%p", window);
+  app->window_id_string = g_strdup_printf ("window:%d", meta_window_get_stable_sequence (window));
 
   _shell_app_add_window (app, window);