Basic entity behavior

[mworzala]

Introducing basic entity behavior trees. For now, they will only support sequential and very basic conditional decision making (eg start following a player if it is nearby).

The system will have support for Hydrazine, EnoriaPF, and my own experimental (and incomplete) pathfinder. Each one has problems and may be better in one case than another. For example, Enodia supports swimming, whereas mine currently does not. But Enodia does not support jumping and cliff avoidance, whereas mine does.