Implemented EXP sharing based on damage contribution

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EXP on kill is now shared between damage contributors. A "damage contributor" is either an individual entity, or a group - depending if the attacker is in a group. This means that not only does the "killing blow" no longer get 100% of EXP, but multiple groups and individuals all receive their fair share of EXP on death (assuming they are still within range of the entity when it dies).

Damage from a given individual or group only counts towards a kill for 10 minutes since that individual or group's last damage to the entity - after this period their damage contribution is removed. This avoids the list of damage contributors growing excessively large for an entity that does a lot of combat but never dies.

EXP sharing within groups is unchanged - the difference is simply that the input to this calculation may be less than 100% of the base EXP reward for the kill if other individuals or groups contributed damage.

Partially addresses #1341 and #1385