Implement entity relationships in ECS

[benreid24]

Entity id should become 64 bits. Use the top 32 for metadata (version id, etc), and the bottom 32 as the index.

Parenting

Implement parenting at the ECS level. Parent-child relationships should be quickly query-able and usable by components/systems. Deleting a parent should also delete all children.

Entity -> parent: Map in vector using entity as index

Parent -> children: Entity-indexed vector stores starting index into vector of child ids. Child vector nodes store child id and next index (essentially a linked list). Minimizes allocations while maintaining O(1) add and remove.

Provide templated base class for components:

template<typename T>
struct ParentAware {
    void parentSet(T* parent);
    void parentRemoved();
};

Dependencies

Add information allowing entities to depend on other entities. Block deletion (unless forced) of entities with dependencies. Allows for reference counted relationships in the ECS natively. Example: Slideshow -> AnimationPlayer. Use similar data approach as parenting.