[BUGFIX] FormEngine suggest wizard in flex form sections

The suggest wizard does not work in FormEngine flex form
section containers for existing containers.

The patch splits the ajax controller action and the FormEngine
wizard display into two classes to have a dedicated entry point
for the ajax action and refactors the flex form field configuration
search logic to find the correct configuration for existing
containers, too.

Change-Id: Ia870618629943b1f3c665cb3a10a7f3769a02540
Resolves: #78558
Releases: master
Reviewed-on: https://review.typo3.org/50839


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Valentin Funk <valentin.funk@computerfabrik.de>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>