Ability to get/set an item in a menu using its index

[lenormf]

Is your feature request related to a problem? Please describe.
One can get the index of the selected element and the whole list of items, as well as set this entire list back into the menu, but there are no methods to get/set an element by index.

Describe the solution you'd like
Implementations of get_item_index(index) and set_item_index(index, item).

[lenormf]

One issue here would be that with checkbox menus, the setter would potentially modify distinct items, c.f. #140.