Add Entity::ListensTo(message_t message).

[mjstahlberg]

An Entity::ListensTo(message_t message) method would be nice to check if a message would be handled without actually sending the message. An usage example would be missiles which should bounce exactly when they can't damage the target that they hit. Right now I'm checking for the presence of a HealthComponent to achieve this but this would break when other entities without a HealthComponent would want to react to the damage that the missile could do. So just like HasComponents, this can be used to find out certain characterstics of the entity at hand.