When checking visibility, we now use the Domains more extensively.
The base rules are:
* If an entity is a child of a domain-less entity, visiblity for the
child is the same as for the parent.
* If an entity is a child of a domain-holding entity, visibility is
determined by the Domain.
* Entities cannot observe things above a Domain.
* Entities can observe things below a Domain, if the Domain allows it.
These rules are not yet set in stone. We need some more time figuring out
exactly how we want visibility implemented. We're striving for a balance
between a powerful but not too complex system, with good performance.
