[FIX] models: in _add_field(), set the field as an attr before setting it up

[FIX] models: in _add_field(), set the field as an attr before setting it up

In the case of custom fields, the field's parameters were set up without the
field being present in the class hierarchy.  Because of this, the parameter
inheritance mechanism was missing the field itself.  As a consequence, custom
selection fields ended up without selection, for instance :-/