Use the maintained AyatanaAppIndicatorGlib 2.0 indicator when it is installed, mirroring the existing Gtk.Menu into a Gio.Menu whose actions keep stable, never-reused names and fire the original items. Fall back to the GTK 3 indicators otherwise; the AppImage stays on its bundled GTK 3 indicator, and the .deb recommends the glib typelib. Fixes #1013