[TASK] Use SpriteIcon for Add icon in Scheduler

Resolves: #61395
Releases: 6.2, 6.3
Change-Id: If77a7b9163ef23d291ca6106c453da059fe5ff21
Reviewed-on: http://review.typo3.org/32621
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>