[BUGFIX] FormEngine fix inline paddings and hierarchy

Fix inline paddings in FormEngine main sections. Also define FormEngine
".checkbox" to be  constrained within FormEngine. Brings improved header
palette hierarchy by setting <h3>.

Resolves: #61672
Releases: master
Change-Id: I9ae611ff60b282d10a0c3b7d102d8426947e043e
Reviewed-on: http://review.typo3.org/32819
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Markus Klein <klein.t3@reelworx.at>
Tested-by: Markus Klein <klein.t3@reelworx.at>