Add Behaviours and Maneuvers for controlling mobs
Remove velocity from Entity; use Behaviour instead

Behaviours (which provide Maneuvers) now tell Mobs how to move, removing the necessity to have a single velocity vector. Projectile implements this pattern with a simple Behaviour that provides a simple Maneuver.

Removed KillBot (not necessary yet and introducing too much complexity to a refactor) and BallisticProjectile (same reasons).