If you specify `Gtk::DialogFlags, Gtk::MessageType, Gtk::ButtonsType` instead of `:int, :int, :int`, FFI will automatically convert symbols like `:modal` or `:destroy_with_parent`.

(The first one is a little tricky, since FFI doesn't support real flags yet; There's a gem that adds support for that, but I haven't gotten 'round to adding it to GirFFI yet).