Created a BT structure for character AI. Still pretty rough (especially
tree creation) and missing some features, but fundamentally seems to
work. Currently, characters walk around the world randomly, and if they
see another character they move towards them. Currently it only tests
for other characters in the FOV once a move task has finished however,
so an interruptable task type should be added next.

In theory any number of agents can share one tree since all
instance-specific memory is contained in AIComp.