volume: Add smooth scrolling to adjust output volume

Allow users to smoothly scroll on the volume indicator icon
to adjust the volume. Do this by simply passing the scroll
event to the slider inside the menu.

https://bugzilla.gnome.org/show_bug.cgi?id=687573

js/ui/popupMenu.js

@@ -636,7 +636,7 @@ const PopupSliderMenuItem = new Lang.Class({
         return true;
     },
 
-    _onScrollEvent: function (actor, event) {
+    scroll: function(event) {
         let direction = event.get_scroll_direction();
         let delta;
 
@@ -655,10 +655,15 @@ const PopupSliderMenuItem = new Lang.Class({
         }
 
         this._value = Math.min(Math.max(0, this._value + delta), 1);
+
         this._slider.queue_repaint();
         this.emit('value-changed', this._value);
     },
 
+    _onScrollEvent: function(actor, event) {
+        this.scroll(event);
+    },
+
     _motionEvent: function(actor, event) {
         let absX, absY;
         [absX, absY] = event.get_coords();