Fix GtkApplicationWindow menubar placement

Since the menubar is part of the content, we need to give
it the content_window as parent window, to make things
work again.
https://bugzilla.gnome.org/show_bug.cgi?id=696561

gtk/gtkwindow.c

@@ -11271,3 +11271,9 @@ ensure_state_flag_backdrop (GtkWidget *widget)
 
   gtk_widget_queue_draw (widget);
 }
+
+GdkWindow *
+_gtk_window_get_content_window (GtkWindow *window)
+{
+  return window->priv->content_window;
+}