Refactor show()/hide() sequences

We seem to have a lot of code that does something along the lines of:

    if (condition)
        actor.show();
    else
        actor.hide();

ClutterActor already has such a thing for exactly this purpose: the 'visible'
property. Use it instead of the mess above.

https://bugzilla.gnome.org/show_bug.cgi?id=672272

js/ui/status/volume.js

@@ -149,13 +149,9 @@ const Indicator = new Lang.Class({
                 }
             }
         }
-        if (showInput) {
-            this._inputTitle.actor.show();
-            this._inputSlider.actor.show();
-        } else {
-            this._inputTitle.actor.hide();
-            this._inputSlider.actor.hide();
-        }
+
+        this._inputTitle.actor.visible = showInput;
+        this._inputSlider.actor.visible = showInput;
     },
 
     _volumeToIcon: function(volume) {