[TASK] Cleanup DataHandler - again and again

This patch straightens several areas to use proper API calls,
and separates bigger methods into smaller methods.

Resolves: #82778
Releases: master
Change-Id: Ibfa80589a026966f4cbd0f2e6e7e4062033b3ab8
Reviewed-on: https://review.typo3.org/54408


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Susanne Moog <susanne.moog@typo3.org>
Tested-by: Susanne Moog <susanne.moog@typo3.org>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>