Handle ChildOf relations on Collisions

[RiscadoA]

Problem

Currently collisions with a child of an entity won't be considered collisions with the parent entity. This means that we're effectively limited to a single collider per parent.

Solution

I'm not sure, but ig that if a collider is a child of another collider, if an external entity collides with the child's collider, the CollidingWith relation should be added to the parent entity? Collisions between children entities should also be possible.