Handle 'remain' corpses as discrete entities

[Deiz]

Per IRC discussion, the current way 'remain' corpses are handled presents several issues. Currently this behaviour is only used for huntable animals.

When units with the 'remain' property in cmpHealth die, they simply play the death animation but continue to exist. UnitAI makes this work by overriding a number of movement-related functions, so that corpses don't start moving around.

Absent an ownership change to Gaia, players that own these animals will have LOS from the corpses until the resources are harvested, and enemy buildings will fire on the corpses until they're destroyed.

Ideally, the promotion logic should be genericized and implemented inside each component, so that unit promotion, gate conversion, and units dying and becoming resources all share common code.