Cleans up unused members of World::Actor.
Adds respond() method to World::Actor and enqueues events.

The Actor class now enqueues collision events and can dequeue all
events. It does not execute code for the ruleset yet.
Adds respond() to World::RuleSet and wires up Rule.

This code will allow RuleSet to trigger event response functions tied to
rules. Actor code will now run the RuleSet for any new events.