cleanup: Replace signal connections with virtual functions

Inheriting from actors allows to use virtual functions instead of signal
connections for multiple cases, so just use them when possible.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/559

js/ui/shellMountOperation.js

@@ -71,11 +71,9 @@ var ListItem = GObject.registerClass({
         let labelBin = new St.Bin({ y_align: St.Align.MIDDLE,
                                     child: this._nameLabel });
         layout.add(labelBin);
-
-        this.connect('clicked', this._onClicked.bind(this));
     }
 
-    _onClicked() {
+    vfunc_clicked() {
         this.emit('activate');
         this._app.activate();
     }