Minimal relation support

[Danon5]

It seems that there is currently no way to defer entity relation changes without a custom command buffer system. Additionally, there is no way to track relation changes via events or the EventRecorder. As of now, relations are almost useful, but these are some big caveats to have. If these features were in place, relations would be extremely useful for something like collisions. Having a collision enter and exit based on relation add / remove would be an effective way to tackle the problem.

[friflo]

When implementing Simplify command buffer I can also add support to defer entity relation changes.

The topic events for add/remove relations is a different feature.
Similar to OnComponentChanged there would be an OnRelationsChanged.