Improve lists of Countries, States and Provinces

Adds the events `on_get_countries_list` and `on_get_states_provinces_list` that allow customizing Countries, States and Provinces lists. (And, as a bonus, adds the list of Italian Provinces ;) ).

This pull request supersedes pull requests https://github.com/concrete5/concrete5/pull/899 , https://github.com/concrete5/concrete5/pull/903, https://github.com/concrete5/concrete5/pull/909, https://github.com/concrete5/concrete5/pull/917