Event Target set operations (agent)

[curds01]

The EventTarget defines a set of objects to serve as the target of an event result. Each target creates a
set. It would be good to be able to combine those with set operators (union, intersection, difference, and negation). This would require two things:

1. The obvious introduction of new AgentEventTarget types implementing those operators.
2. Upgrade of parsing in the EventSystem. Currently, the parsing assumes that all targets are immediate children of the <EventSystem> tag. The set operators will have nested target tags. The parsing needs to handle this and reconcile names.

More generally, the idea of defining a set of agents is more generally useful -- such as defining agent goals #142). The core functionality is the same -- given the world of agents, create a subset based on arbitrary criteria. That same functionality should be available to both events and other mechanisms without duplicating code. This more general idea probably requires its own issue.