Extract all admin panel code to an own extension to
reduce cross-dependencies and allow clearer scoping.

See Epic / Initiative for background.

Related: #84044
Resolves: #84159
Releases: master
Change-Id: I0f6e2643746c94f0e10b87146ca7e86cbf8bc112
Reviewed-on: https://review.typo3.org/56030


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Mathias Schreiber <mathias.schreiber@typo3.com>
Tested-by: Mathias Schreiber <mathias.schreiber@typo3.com>
Reviewed-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>
Tested-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>