[MOD][18.0] hr_employee_language: languages selection

[mymage]

As in #1486
Changes the source of language list moving it from the res.lang.csv file to res.lang model.
In this way if you add a language at runtime it will be immediately available.

Added _compute_display_name (ex name_get) to show language name instead language code on selection or m2o widget

[mymage]

@dreispt @primes2h @TheMule71 plase review.
Little differences from #1512

[mymage]

@dreispt Thanks for your notes; I'm not so skilled on coding so your suggestion is very appreciated. I'll give a look asap.

[mymage]

@dreispt Thanks to your comments I refactored the code. The only "strange" thing that I made is this:
self_data = self.env["hr.employee.language"].search([]) for record in self_data:
to have CodeCov at 100%: this because despite I create an element in hr.employee.language for test, here self is empty.