[BUGFIX] Decrease z-index for disabled fields in FormEngine

The z-index for disabled input fields is decreased to a much lower value
to avoid overlaying the module header bar.

Resolves: #80428
Releases: master, 7.6
Change-Id: Ia6f861fc66269b843571720e31f4f935d5cc9614
Reviewed-on: https://review.typo3.org/52149


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Frank Nägler <frank.naegler@typo3.org>
Tested-by: Frank Nägler <frank.naegler@typo3.org>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>