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/slider.js

@@ -19,7 +19,8 @@ var Slider = GObject.registerClass({
             style_class: 'slider',
             can_focus: true,
             reactive: true,
-            accessible_role: Atk.Role.SLIDER
+            accessible_role: Atk.Role.SLIDER,
+            x_expand: true,
         });
 
         this._releaseId = 0;