actor: Warn on adding/removing itself as a child
actor: Warn on adding/removing itself as a child

ClutterActor should warn if a user tries to add or remove an actor to,
and from, itself on the scene graph.

Clutter will likely crash, or warn way down the line, but if we can make
debugging simpler then we should.

GNOME/clutter,GNOME/clutter,GNOME/clutter,GNOME/clutter,GNOME/clutter