slider: Make clicking anywhere on the slider menu item pass to the slider

This is a regression from splitting the slider out that never got fixed.
Restore the previous (useful) behavior by adding a public API to the
slider that lets us pass an event through.

https://bugzilla.gnome.org/show_bug.cgi?id=704368

js/ui/slider.js

@@ -98,6 +98,10 @@ const Slider = new Lang.Class({
     },
 
     _startDragging: function(actor, event) {
+        this.startDragging(event);
+    },
+
+    startDragging: function(event) {
         if (this._dragging) // don't allow two drags at the same time
             return false;
 
@@ -112,7 +116,6 @@ const Slider = new Lang.Class({
         let absX, absY;
         [absX, absY] = event.get_coords();
         this._moveHandle(absX, absY);
-
         return true;
     },
 

js/ui/status/volume.js

@@ -43,6 +43,10 @@ const StreamSlider = new Lang.Class({
         this.item.addActor(this._icon, { align: St.Align.MIDDLE });
         this.item.addActor(this._slider.actor, { expand: true });
 
+        this.item.actor.connect('button-press-event', Lang.bind(this, function(actor, event) {
+            this._slider.startDragging(event);
+        }));
+
         this._stream = null;
     },