[FEATURE] Module menu: System group

On many installation the module menu is very crowded.
In order to distribute backend modules on module menu
groups equally a new module menu group is added:

Adds "System" as module menu group.

The new group "System" hosts low-level status tools
and does not bring a navigation tree:

* user management
* install
* scheduler
* log
* lowlevel
* reports

This restructuring is a step on our way to backend modules
that themselves steer the nav frame.

Change-Id: I70c345848d404337d00c51d678e03d619a0190b8
Resolves: #48625
Related: #5128
Releases: 6.2
Reviewed-on: https://review.typo3.org/21177
Reviewed-by: Georg Ringer
Tested-by: Georg Ringer
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring
Reviewed-by: Ernesto Baschny
Tested-by: Ernesto Baschny