[TASK] Show backend_layout fields also for sys folders

Even though folders are not visible in the frontend it is useful to have
the backend layouts available for this page type.

Change-Id: I1bab2ea98a9afa0d62ac2a10a907d634a4e59536
Resolves: #53992
Releases: master
Reviewed-on: http://review.typo3.org/43107


Reviewed-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Markus Klein <markus.klein@typo3.org>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>