panel: allow padding around panel buttons to shrink

for narrow screens (eg, portrait orientation)

https://bugzilla.gnome.org/show_bug.cgi?id=651299

js/ui/userMenu.js

@@ -364,7 +364,7 @@ UserMenuButton.prototype = {
     _init: function() {
         PanelMenu.Button.prototype._init.call(this, 0.0);
         let box = new St.BoxLayout({ name: 'panelUserMenu' });
-        this.actor.set_child(box);
+        this.actor.add_actor(box);
 
         this._lockdownSettings = new Gio.Settings({ schema: LOCKDOWN_SCHEMA });