[FEATURE] Scheduler: Add delete button to edit view

The task edit view misses a delete button similar to the tceforms edit
view.
Add this button for the edit action.

Group the buttons to distinguish close from save and save from delete.

Resolves: #48054
Releases: 6.2
Change-Id: I79997357f1069c53398f813fcd3b66f809fc059e
Reviewed-on: https://review.typo3.org/20860
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn