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/bluetooth.js

@@ -106,10 +106,7 @@ const Indicator = new Lang.Class({
             /* TRANSLATORS: this means that bluetooth was disabled by hardware rfkill */
             this._killswitch.setStatus(_("hardware disabled"));
 
-        if (has_adapter)
-            this.actor.show();
-        else
-            this.actor.hide();
+        this.actor.visible = has_adapter;
 
         if (on) {
             this._discoverable.actor.show();