actor: Make Actor.add_child and Container.add_actor idempotent

actor: Make Actor.add_child and Container.add_actor idempotent

And make sure that overriding Container and calling
clutter_actor_add_child() will result in the same sequence of operations
as the current set_parent()+queue_relayout()+signal_emit pattern.

Existing containers can continue using:

        clutter_actor_set_parent (child, CLUTTER_ACTOR (container));
        clutter_actor_queue_relayout (CLUTTER_ACTOR (container));
        g_signal_emit_by_name (container, "actor-added", child);

and newly written containers overriding Container.add() can simply call:

        clutter_actor_add_child (CLUTTER_ACTOR (container), child);

instead.