Add option to expand and collapse all children of segment group

[dieknolle3333]

Dev instance

https://collapseallsegmentgroupchildren.webknossos.xyz/

Steps to test:

• go to segments tab in any annotation
• click some segments and create some groups, groups should be nested for at least three levels
• right click any group to access group context menu
• if the segment group has any subgroups, there should be one or two new items: "expand/collapse all subgroups".
• if the group is expanded and some subgroups are expanded and some are collapsed, both entries should be shown
• if the group itself is expanded and all subgroups are collapsed, you should only be able to expand them. the opposite is true if all subgroups are expanded.
• if the group itself is collapsed, you should only be able to expand all subgroups. the group itself will be expanded aswell.
  - # 7827: Focus in segment list should also work for segments in a collapsed group

TODOs:

• icon
• switch label
• default: all expanded
• recursively expand subgroups

Issues:

• fixes Collapse subgroups under any higher order group in the segment list #7829

---

(Please delete unneeded items, merge only when none are left open)

• Updated changelog
• Updated migration guide if applicable
• Updated documentation if applicable
• Adapted wk-libs python client if relevant API parts change
• Removed dev-only changes like prints and application.conf edits
• Considered common edge cases
• Needs datastore update after deployment

[dieknolle3333]

I decided to work on #7827 here because I need the changes that are made in this PR and I think it's only a small change to solve this issue.

[philippotto]

cool, looks good overall! just see my few comments.

[philippotto]

nice, works well 👍