Signals
-------
	There are three types of signals in Angel Wars.  "Actor to Actorlist"
	is a char * returned by an Actor's update() method, which tells the
	Actorlist what to to with the Actor.  This is mainly (actually, ONLY)
	used so an Actor can tell the list when it supposed to be destroyed.
	(The "suicide" signal.)  "Actor to Actor" signals are sent from one
	actor to another, usually when they collide.  (When the Actorlist finds
	two of its Actors in collision, it gets a signal from each involved
	Actor's collided() method and sends it to the other.)  The is mostly
	used for an actor to cause damage to another actor.  Actor to Actor
	signals can also be sent explicitly by invoking the receiving actor's
	get_signal() method.  "Behavior to Actor" signals are simply a
	character pointer returned by the behavior to its owning actor.  This
	is mostly used by BehaviorFromKeyboard to tell its owner what keys
	have been pressed, but could have other applications.  (Like, for
	example, enemies that fire projectiles.)

Signal Reference
----------------

Actor to Actorlist
	"suicide"
		remove the actor from the list, deallocating if necessary

Actor to Actor
	"hurt"
		try to damage the actor
	"enemy_killed"
		the enemy pointed to in "source" has been killed

Behavior to Actor
	"fire"
		attack with the current weapon
	"next_weapon"
	"prev_weapon"
	