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/viewSelector.js

@@ -130,7 +130,10 @@ var ViewSelector = GObject.registerClass({
     }
 }, class ViewSelector extends Shell.Stack {
     _init(searchEntry, showAppsButton) {
-        super._init({ name: 'viewSelector' });
+        super._init({
+            name: 'viewSelector',
+            x_expand: true,
+        });
 
         this._showAppsButton = showAppsButton;
         this._showAppsButton.connect('notify::checked', this._onShowAppsButtonToggled.bind(this));