[TASK] Introduce AbstractFormElement for FormEngine elements

This is a follow up to #61958 which improves the code structure.

Resolves: #62011
Related: #61958
Releases: master
Change-Id: Ia89be0cfaa0779414652e5f5791b3920aa0d9927
Reviewed-on: http://review.typo3.org/33115
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Benjamin Mack <benni@typo3.org>
Reviewed-by: Alexander Opitz <opitz.alexander@googlemail.com>
Tested-by: Alexander Opitz <opitz.alexander@googlemail.com>
Reviewed-by: Markus Klein <klein.t3@reelworx.at>
Tested-by: Markus Klein <klein.t3@reelworx.at>