Add inverse method to product showed_name and use it in the form view

CHANGELOG:

### Changed
- [#698](https://gitlab.com/coopdevs/odoo-somconnexio/-/merge_requests/698) Use the showed name to define the Product name.

Related card: https://trello.com/c/lXP8e8vE/1025-treure-restriccions-de-categories-de-producte-en-el-codi

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