Split gamma into multiple files

Moved game mode, behaviour type and behaviour functions into their own
files and made their global storage variables visible only in there.
Added functions to access the variables.
Added function overrides for behaviours

Added native Gamma_AddBehaviourFunctionOverride, can only be called
during Gamma_OnCreateBehaviour
Added forward Gamma_OnCreateBehaviour
Added forward Gamma_OnBehaviourCreated
Added forward Gamma_OnDestroyBehaviour
Added forward Gamma_OnBehaviourDestroyed
Added stocks to set override for the built-in behaviour fowards
Moved the forwards/natives a bit around in gamma.inc
Removed the hard limit of one behaviour per plugin per game mode