substitute _render_attrs by values_and_attributes method, keeping bw compat

replace form_field_[encoding|vocabulary] methods on form by encoding|vocabylary(form) methods on fields

refactor vocabulary handling to avoid having to define methods on form objects, use .choices instead even for non final relations. Details: * deprecated support of <role>_<relation>_vocabulary methods on forms * new utility functions for relations vocabulary * most logic now on the RelationField class

cleanup