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/overviewControls.js

@@ -452,7 +452,7 @@ class ControlsManager extends St.Widget {
         this.add_actor(this._dashSlider);
 
         this._group.add_actor(this._dashSpacer);
-        this._group.add(this.viewSelector, { x_fill: true, expand: true });
+        this._group.add_child(this.viewSelector);
         this._group.add_actor(this._thumbnailsSlider);
 
         layout.connect('allocation-changed', this._updateWorkspacesGeometry.bind(this));