Entities retain relations to removed entities

[opl-]

A relation to another entity isn't removed when the target entity is removed. Entity.getRelationsByKind() continues to return removed entities:

with(geary) {
	val entityA = entity()
	val entityB = entity()

	// Using a random component.
	entityA.addRelation(components.noInherit, entityB.id)
	entityB.removeEntity()

	// Throws .-.
	check(entityA.getRelationsByKind(components.noInherit).isEmpty())
}

This is disastrous if entity id reuse is enabled.

Additionally, no event is emitted on the entity owning the relation, preventing cleanup without listening to all OnEntityRemoved events and iterating over all entities with that relation (obviously slow).

[0ffz]

Reading up a little more I've realized flecs implements entity versioning in a fairly simple way by assuming relations should be removed when an entity is removed. It's still likely a decently sized change but I have a better idea of how to implement both of these features together, so I've marked that issue as a parent.