[TASK] Stabilize / Solidify public API of adminPanel

* public service classes for extension authors have gotten tests
* exceptions in public API have been made more specific
* non-API has been defined as internal
* API has been cleaned up

Resolves: #85675
Releases: master
Change-Id: Icc9bab027eaca07fccd9368763350487959148a7
Reviewed-on: https://review.typo3.org/57714


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>