js: Replace child properties

Every since commit aa394754, StBoxLayout has supported ClutterActor's
expand/align properties in addition to the container-specific child
properties. Given that that's the only container left with a special
child meta, it's time to fully embrace the generic properties (and
eventually remove the child meta).

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/780

js/ui/status/brightness.js

@@ -41,7 +41,7 @@ class Indicator extends PanelMenu.SystemIndicator {
         let icon = new St.Icon({ icon_name: 'display-brightness-symbolic',
                                  style_class: 'popup-menu-icon' });
         this._item.add(icon);
-        this._item.add(this._slider, { expand: true });
+        this._item.add_child(this._slider);
         this._item.connect('button-press-event', (actor, event) => {
             return this._slider.startDragging(event);
         });