Listino for Fornitore

Following client's request to let each Client have its own product list
with price and validity date.

A new FornitoreService has been created to deal with parent child
relationship.

Products management UI has been partly moved into Fornitore's.

New product and edit product form has yet to be implemented.

Business logic to let the user pick only products from a given client
while creating/editing a Viaggio is yet to be implemented.