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