Allow equivalent relations

Issue Summary

It would be very, very great to be able to use equivalent relations.
Atm, it's possible, but the way the links are presented is based on parent/child relations.

At admin screen you can define "Terminology" (Parent links). But these are not needed for equivalent relations.

Example relation: "Peter looks like John". Well, this is (might) be the same as "John looks like Peter". It shouldn't matter which node is the parent nor child. But they should both show this relation equivalently, not separated into different blocks titled with parent resp. child (or some other values you define).