js: Cleanup use of StBin

StBins (inc StButton) should have their content managed via :child
rather than add_child()

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3010>

js/ui/dash.js

@@ -210,7 +210,7 @@ class ShowAppsIcon extends DashItemContainer {
         });
         this.icon.y_align = Clutter.ActorAlign.CENTER;
 
-        this.toggleButton.add_child(this.icon);
+        this.toggleButton.child = this.icon;
         this.toggleButton._delegate = this;
 
         this.setChild(this.toggleButton);