Introduce GroupFunctionGateway, and move some stuff to GroupGateway

What does this MR do?

• each commit message should say so pretty clearly
• please review the commits individually

How confident are you it won't break things if deployed?

Would probably be good to check everything that was touched, especially in the ugly Xhr code... that's not inspiring confidence.

How to test

• reviewing each commit individually
• asking your IDE if problems exist

=> workgroup-function configuration + changes when moving regions/workgroups around

Checklist

• no intended changes, no test
• no unrelated changes
• asked someone for a code review
• set a "for:" label to indicate who will be affected by this change
• use "state:" labels to track this MR's state until it was beta tested
• added an entry to CHANGELOG.md
• Once your MR has been merged, you are responsible to create a testing issue in Beta Testing Repo:
  • Consider writing a detailed description in German.
  • Describe in a few sentences, what should be tested from a user perspective.
  • Also mention different settings (e.g. different browsers, roles, ...). how this change can be tested.
  • Be aware, that also non technical people should understand.