Release notes for GAP 4.9

[fingolfin]

Below is a list of pull requests which are NOT in stable-4.8, formatted as a task list.

If you want to help, do this: Take any PR which has not been "ticked" yet. Look at it, and decide whether it contains changes that should be mentioned in the release notes. If yes, do so.
In either case, once you are done reviewing the PR, tick it, so that nobody else has to look at it.

TODO: explain how / where to add the release notes... perhaps on a Wiki page, or a HackMD file?

UPDATE: turns out that clicking the checkboxes is veery slow on GitHub. I will look into a way to first reduce the list some more...

[olexandr-konovalov]

The task of walking through all PR merged to master but not into stable is quite tedious. We are doing that in a shared document - if anyone wants to help, ask one of the assignees for this issue for details.

When we will have that completed, we can update release notes on Wiki and add the new chapter to the doc/changes manual.

To make collecting release notes easier, @fingolfin created new label "not-for-release-notes". Otherwise, if the PR should be mentioned in release notes, please use one or several more specific labels, e.g. "enhancement", "new feature" etc.

[olexandr-konovalov]

@fingolfin we certainly need to update the list of PRs at hackmd for that...

[fingolfin]

I cand do taht eventually, but in the meantime, there is plenty of work already there, no? And I am reluctant to invest any work into it unless I know it won't be wasted