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/workspacesView.js

@@ -844,10 +844,7 @@ const WorkspacesDisplay = new Lang.Class({
                         if (!primaryView)
                             return;
                         primaryView.actor.opacity = opacity;
-                        if (opacity == 0)
-                            primaryView.actor.hide();
-                        else
-                            primaryView.actor.show();
+                        primaryView.actor.visible = opacity != 0;
                     }));
         }));
     },