Called when an actor enters or leaves a trigger.
Only the selected actor will trigger a trigger unless the trigger has the
any_actor = YES flag set.
If trigger object has a
trigger(actor, state) function, then it is called passing the actor and a state of
TRIGGER_ENTER or
TRIGGER_LEAVE.
Otherwise, if the trigger object has a
triggerEnter() or a
triggerLeave() function those will be called.
And finally, it will try and call the code passed during a
addTrigger() call.