Added ExternalEventTrigger; a trigger object that can be exposed to external code to trigger events.
Added EventEffectAgentState; an event effect that will change the state of target agents.
Added TargetAgentById; an event target for a single agent identified by its unique id.
Augmented the viewer to detect external triggers and connect keyboard and mouse events to them. For details see src/Menge/MengeVis/Runtime/EventInjectContext.h
Added new example exercising this functionality: examples/core/userEvent.xml. A single agent placed in a diamond-shaped environement. The arrow keys control what direction the agent moves (one of the four cardinal directions).
Augmented the event system.