window: Delay showing auto mnemonics on focus in

Just as in ed7a417dcb we don't want to
show auto mnemonics immediately but only after a short delay. In
particular this allows to capture screenshots without visible
mnemonics.

https://bugzilla.gnome.org/show_bug.cgi?id=684517

gtk/gtkwindow.c

@@ -6248,7 +6248,7 @@ maybe_set_mnemonics_visible (GtkWindow *window)
                                 NULL, &mask);
           if (window->priv->mnemonic_modifier == (mask & gtk_accelerator_get_default_mod_mask ()))
             {
-              gtk_window_set_mnemonics_visible (window, TRUE);
+              _gtk_window_set_auto_mnemonics_visible (window);
               break;
             }
         }