meta-background-actor: Chain up in dispose

This was preventing Clutter from running its dispose, preventing
it from being destroyed correctly. While a major bug and possibly
big memory leak, the most obvious effect was the message that Clutter
spat out:

  Clutter-CRITICAL **: clutter_actor_iter_next: assertion `ri->age == ri->root->priv->age' failed

Since we never disposed the actor correctly, it was never removed
from its parent, and the age wasn't updated correctly.