appMenu: Add separator between "Details" and "Quit"

The two actions are sufficiently different to warrant their own
groups.

https://gitlab.gnome.org/GNOME/gnome-shell/issues/968

js/ui/panel.js

@@ -105,6 +105,8 @@ class AppMenu extends PopupMenu.PopupMenu {
             });
         });
 
+        this.addMenuItem(new PopupMenu.PopupSeparatorMenuItem());
+
         this.addAction(_("Quit"), () => {
             this._app.request_quit();
         });