Widget of "large autocompletion list" : combine autocomplete and list

[tfrancart]

Sometimes the list of autocompletion proposals is too small. We want the user to be able to discover all possible values, showing only 10 is too restrictive.
This was pointed out in #518
We could enlarge the list of autocompletion proposals a little, but this will not solve the problem completely.

What we would like to do is to have a search field, that would return a long list of results, that can be scrolled in a dropdown. A combination of autocompletion and list widget.

Select2 supports this: https://select2.org/data-sources/ajax (which could make it work for both list and autocomplete widgets ?)