Merge in code from github D8 ports/forks

Problem/Motivation

There are at least two D8 versions of this on github:

- https://github.com/Sebastix/list_predefined_options
- https://github.com/Realityloop/list_predefined_options

These are strictly speaking ports rather than forks, as they don't share any git history with this module. They also predate the creation of the D8 version here.

The Sebastix version is ahead of Realityloop.

Steps to reproduce

Proposed resolution

We should see if there is any useful code in these forks and merge it into the code here.

Remaining tasks

User interface changes

API changes

Data model changes