[TASK] Use fixed font for config of backend_layout

Use a fixed font + the possibility for using the the tab character in
the config field of backend_layouts.

Resolves: #72769
Releases: master
Change-Id: If535aff04ab1685f4b70dbf241ce6cbf70356e5c
Reviewed-on: https://review.typo3.org/46016


Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Andreas Fernandez <typo3@scripting-base.de>
Tested-by: Andreas Fernandez <typo3@scripting-base.de>