util: Use explicit transition for fading actors

As strongly suggested by ebassi on IRC:

<ebassi> [19:34:50] zeenix: and I would *strongly* suggest you use a
proper ClutterTransition in this case, instead of relying on implicit
animations, since you're controlling the start and end states, and you
want to get notifications

Besides being the right thing to do here, this also works around the issue of
Boxes unable to change the actor visibility at the end of transition and
avoid the warning on console about transition being null against clutter
1.16 in certain cases.

https://bugzilla.gnome.org/show_bug.cgi?id=700306