actor: Make consistent use of the name in error reporting

actor: Make consistent use of the name in error reporting

Whenever we are warning inside ClutterActor we prefer the actor's name
to its type, if the name is set. The current code is made less readable
by the use of the ternary operator:

  priv->name != NULL ? priv->name : G_OBJECT_TYPE_NAME (self)

This looks like a job for a simple convenience function.