[FEATURE] Description-field for scheduler-jobs

Adds a description-textfield for tasks. This will be displayed
in the scheduler-list. The description can either be shown as
hover (default) or below the job-name, configurable through
the extension-settings for "scheduler".

Change-Id: If0191da43677fc261cb73257e3ede848a5375d69
Resolves: #52696
Releases: 6.2
Reviewed-on: https://review.typo3.org/24610
Reviewed-by: Philipp Gampe
Tested-by: Philipp Gampe
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring