Fix key navigation on system menu sliders

Rely key press events management if the menu item contains
a slider.

https://bugzilla.gnome.org/show_bug.cgi?id=706386

js/ui/status/volume.js

@@ -44,6 +44,9 @@ const StreamSlider = new Lang.Class({
         this.item.actor.connect('button-press-event', Lang.bind(this, function(actor, event) {
             this._slider.startDragging(event);
         }));
+        this.item.actor.connect('key-press-event', Lang.bind(this, function(actor, event) {
+            return this._slider.onKeyPressEvent(actor, event);
+        }));
 
         this._stream = null;
     },