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.