[Decision] Commit function

[flo91]

This PR is for working on the decision about the commit function (communicating the current phase to plugins).

It should at least reach the state "solutions clear" before closing this PR.

Basics

• [x] Short descriptions of your changes are in the release notes (added as entry in doc/news/_preparation_next_release.md which contains _(my name)_) Please always add something to the release notes.
• [x] Details of what you changed are in commit messages (first line should have module: short statement syntax)
• [ ] References to issues, e.g. close #X, are in the commit messages.
• [ ] The buildservers are happy. If not, fix in this order:
  • [ ] add a line in doc/news/_preparation_next_release.md
  • [ ] reformat the code with scripts/dev/reformat-all
  • [ ] make all unit tests pass
  • [ ] fix all memleaks
• [x] The PR is rebased with current master.

Checklist

• [ ] I added unit tests for my code
• [ ] I fully described what my PR does in the documentation (not in the PR description)
• [ ] I fixed all affected documentation (see Documentation Guidelines)
• [x] I fixed all affected decisions (see Decision Process)
• [ ] I added code comments, logging, and assertions as appropriate (see Coding Guidelines)
• [ ] I updated all meta data (e.g. README.md of plugins and METADATA.ini)
• [ ] I mentioned every code not directly written by me in reuse syntax

Review

• [ ] Documentation is introductory, concise, good to read and describes everything what the PR does
• [ ] Examples are well chosen and understandable
• [ ] Code is conforming to our Coding Guidelines
• [ ] APIs are conforming to our Design Guidelines
• [ ] Code is consistent to our Design Decisions

Labels

• [ ] Add the "work in progress" label if you do not want the PR to be reviewed yet.
• [ ] Add the "ready to merge" label if the basics are fulfilled and no further pushes are planned by you.