Added WithDeathAnimation

Deprecated RenderInfantry's SpawnsCorpse logic, renamed CrushableInfantry to Crushable (it can now be used to make other actors crushable too, afterall).

This can be used to replace DeadBuildingState, too, once #6389 is fixed.

For corpses rather than death animations, read the comments on #4163 about using LeavesHusk for that.
Can you add some `[Desc("")]` labels for https://github.com/OpenRA/OpenRA/wiki/Traits while you are at it?