utils: Request a modal dialog when transient_for is set

https://bugzilla.gnome.org/show_bug.cgi?id=684306

src/core/util.c

@@ -640,15 +640,17 @@ meta_show_dialog (const char *type,
       tmp = tmp->next;
     }
 
-  g_ptr_array_add (args, NULL); /* NULL-terminate */
-
   if (transient_for)
     {
       gchar *env = g_strdup_printf("%d", transient_for);
       setenv ("WINDOWID", env, 1);
       g_free (env);
+
+      append_argument (args, "--modal");
     }
 
+  g_ptr_array_add (args, NULL); /* NULL-terminate */
+
   g_spawn_async (
                  "/",
                  (gchar**) args->pdata,