quickSettings: Fix slider accessibility

Since we put the actual slider into a bin to get a proper focus
indication, the slider isn't focused anymore and its accessible object
is therefore invisible to the screen reader.

Fix this by passing the slider's accessible object to the actor
that takes the focus.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2431>

js/ui/quickSettings.js

@@ -195,6 +195,7 @@ var QuickSlider = GObject.registerClass({
         });
         box.add_child(sliderBin);
 
+        sliderBin.set_accessible(this.slider.get_accessible());
         sliderBin.connect('event', (bin, event) => this.slider.event(event, false));
 
         this._menuButton = new St.Button({