Merge: Introduce model_visitor

As requested, this is a basic visitor framework on models.
Services names are inspired from those in the AST visitor.

Fell free to comment the API or suggest ideas.

Pull-Request: #1776
Reviewed-by: Romain Chanoir <romain.chanoir@viacesi.fr>
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>