Added 3 new natives

Added Gamma_BehaviourTypeHasBehaviours to check if a behaviour type has
any behaviours registered
Added Gamma_GetRandomBehaviour to get a random behaviour of a behaviour
type
Added Gamma_GiveRandomBehaviour to give a random behaviour to a client,
it also returns the given behaviour
Modified bossfightfortress to use those additions
Modified game-mode_template to use those additions