[ShellApp] Ignore removal of windows we're not interested in

Previously shell_app_remove_window assumed that it was being
passed a window in its list; rather than having callers check
whether a window is interesting and only if so removing it
from the app, just ignore removal of windows we aren't interested
in, like how we ignore addition of windows we already have.

https://bugzilla.gnome.org/show_bug.cgi?id=598502

src/shell-app.c

@@ -324,6 +324,9 @@ void
 _shell_app_remove_window (ShellApp   *app,
                           MetaWindow *window)
 {
+  if (!g_slist_find (app->windows, window))
+    return;
+
   g_signal_handlers_disconnect_by_func (window, G_CALLBACK(shell_app_on_unmanaged), app);
   g_signal_handlers_disconnect_by_func (window, G_CALLBACK(shell_app_on_user_time_changed), app);
   g_object_unref (window);