[CLEANUP] Clean code in FormEngine

\TYPO3\CMS\Backend\Form\FormEngine could use a few cleanups:

* Import some classes to avoid unnecessary fully qualified class names
* Fix some PHPdoc issues
* Slightly adjust some method signatures
  (when 0, 1, or '' were used for boolean information)
* Remove some unused local variables

Resolves: #58995
Releases: 6.3, 6.2
Change-Id: Ic79acf80e75b78f9b8b70c946dd15e5b32996fb2
Reviewed-on: https://review.typo3.org/31016
Reviewed-by: Markus Klein
Tested-by: Markus Klein