Graphviz debug: Enable edges between cluster nodes, but using invisible
dummy nodes and ltail/lhead attributes.
Some fixes and cleanup to allocating and freeing DepsRelations.

(Un-)Registering of relations in node inlinks/outlinks now happens in
constructor/destructor. Eventually it may be preferable to manage
relations in a central mempool in the Depsgraph instead of using shared
ownership in both from and to node.
Removed copy function for relations.

This was not used, and it doesn't make much sense because one would
have to manually modify a copy after creating it, avoiding all the
pitfalls. Relations should just be deleted and replaced by creating a
new relation instead.
Removed leftover next/prev pointers from disused ListBase.