Z3c-relationfield

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* Don't handle ``IObjectModified`` events for objects that do not yet
have a parent. There is no need to do so anyway, as these objects cannot
have outgoing relations indexed.

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* Introduce a ``RelationChoice`` field that behaves like
``schema.Choice`` but tracks relations. In combination with a source
(such as created by ``RelationSourceFactory`` provided by
``z3c.relationfieldui``) this can be used to create drop-down
selections for relations.

* Clarify the way comparing and sorting of ``RelationValue`` objects is
done in order to better support choice support.

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* When a relation is broken, properly re-catalog things.

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* Introduce sensible sort order for relations, based on a
``(from_attribute, from_path, to_path)`` tuple.

* Relations will now never compare to ``None``.

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* Introduce two new interfaces: ``IHasOutgoingRelations`` and
``IHasIncomingRelations``. ``IHasOutgoingRelations`` should be provided
by objects that actually have relations set on them, so that
they can be properly cataloged. ``IHasIncomingRelations`` should be
set on objects that can be related to, so that broken relations
can be properly tracked. ``IHasRelations`` now extends both,
so if you provide those on your object you have an object that can
have both outgoing as well as incoming relations.

* Improve broken relations support. When you now break a relation (by
removing the relation target), ``to_id`` and ``to_object`` become
``None``. ``to_path`` however will remain the path that the relation
last pointed to. ``TemporaryRelation`` objects that when realized
are broken relations can also be created.

You can also for broken status by calling ``isBroken`` on a
relation.

* The signature of the top-level function ``create_relation``
changed. It used to take the object to which the relation was to be
created, but should now get the path (in ``IObjectPath`` terms).
``create_relation`` will now create a broken relation object if the
path cannot be resolved.

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* Added support for ``RelationList`` fields. This allows one to
maintain a list of ``RelationValue`` objects that will be cataloged
like the regular ``Relation`` fields.

* Get rid of ``IRelationInfo`` adapter requirement. Just define a
``create_relation`` function that does the same work.

* When looking for relations on an object be more tolerant if those
cannot be found (just skip them) - this can happen when a schema is
changed.