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

refactor form field value handling, to get a nicer api and an easier algorithm to get field's value Details: * new .typed_value / .display_value on fields * droped form_field_value on form * .value attribute of field instead of .initial * nicer field's __init__, allowing to give a lambda as value's value

missing imports

yams api update

drop form_field_name/form_field_id methods from form object, in favor of field.input_name(form) / field.dom_id(form)

NothingToEdit is not a ValidationError, simplify

fix translation of message to view newly created entity: must be done once language is set...