[TASK] Make the list module nesting optional

The nesting for the list module needs an xclass which is likely to
break. Introduce a switch which allows to disable this feature.

Change-Id: I40778ebd10ebc3e62e3f68ed5c84ec3bf0cb08b8
Reviewed-on: https://review.typo3.org/25136
Reviewed-by: Jo Hasenau
Tested-by: Jo Hasenau