This can be an alternative to the 'clone' action, in particular when the
'edition' form of the entity to be cloned is not readily usable (e.g. needs
tweaking to hide inlined sub-forms, etc.).

This is not ready for inclusion and was backported from client
application to soon.

Thus grouping related things together.

Otherwise the assertion in __registered__() is hit at db-init step.

This method returns a dictionary mapping entity type to composite relation
information allowing to walk the schema graph upstream from these entity
types.

* `parent_relations`/`child_relations` are used to get relation information
  from an entity type.
* `parent_structure`/`child_structure` return the full structure of the
  composite graph, this structure will change later on to conform to the
  "definition" methods.
* `parent_related`/`child_related` yield information about related entities in
  the composite graph either from the parent or child perspective.

These are used to build the IContainer/IContained classes.

Properly walk through relation definitions accounting for parent role and
entity type in iter_schema. Better use Yams API in passing.

This makes selections of CloneEntityHook simpler, cheaper and more robust.
Drop IClonableAdapter.clone property, not longer useful.

This is not used currently.

The dictionary was not really useful, a set is enough. Also sort these
children in `structure()` method for predictability.

Also add the `structure` method which returns the fully evaluated
graph (of schema objects), useful for testing.

Actual tests are not so specific to "entities" despite most of the code
currently lies there and they are not so unit either.

The action url will contain a linkto with <clone_rtype> set for the new (cloned) entity.

And pass them to composite_schema_graph.

Test coverage is currently rather low. A more complex test schema needs to be
created.