Update meta_show_dialog for gtk's removal of --screen

gtk3 no longer has the --screen command-line argument, which mutter
was passing to zenity. Use --display (with an explicitly-specified
screen number) instead.

https://bugzilla.gnome.org/show_bug.cgi?id=643683

src/core/session.c

@@ -1804,7 +1804,7 @@ warn_about_lame_clients_and_finish_interact (gboolean shutdown)
                            "and will have to be restarted manually next time "
                            "you log in."),
                          "240",
-                         meta_screen_get_screen_number (meta_get_display()->active_screen),
+                         meta_get_display()->active_screen->screen_name,
                          NULL, NULL,
                          None,
                          columns,