Allows defining of task-groups and groups tasks in
scheduler-list by those groups.

Tasks inside a hidden group are neither automatically executed
nor displayed in the scheduler module.
Task-groups feature a description field that can hold additional
information and is displayed below the group name in the scheduler
list.

Change-Id: I445cf9296ab03e9e0e9e3a35ba5a6aa66ad23f13
Resolves: #52695
Releases: 6.2
Reviewed-on: https://review.typo3.org/24608
Reviewed-by: Xavier Perseguers
Tested-by: Xavier Perseguers
Reviewed-by: Georg Ringer
Tested-by: Markus Klein
Reviewed-by: Markus Klein