unlockDialog: Turn mainBox into an St.Widget

This is the container that contains the elements that are interesting
for keynav, so it must be aware of focus-chain and -navigation.

https://gitlab.gnome.org/GNOME/gnome-shell/issues/2210

js/ui/unlockDialog.js

@@ -568,7 +568,7 @@ var UnlockDialog = GObject.registerClass({
             this._otherUserButton, 'visible', Gio.SettingsBindFlags.GET);
 
         // Main Box
-        let mainBox = new Clutter.Actor();
+        let mainBox = new St.Widget();
         mainBox.add_constraint(new Layout.MonitorConstraint({ primary: true }));
         mainBox.add_child(this._stack);
         mainBox.add_child(this._notificationsBox);