Implement a triggered-ability notification framework

[asilano]

Cards like Scheme and Prince are a bit hacked in at present. In particular, those two are interdependent, but that's hard to manage.

A proper notification framework to allow triggered abilities may ease that.

Consider wisper

[asilano]

Probably not wisper, because I need a central marshaller to make sure events are performed on each player in turn, and each player can choose the order.