[TASK] Scheduler: Add "Add Task" button to header

This patch adds a "Add Task" button to the left area of
the module header.
It improves usability for long task lists and enforces
the standard layout of backend modules.
The "Add Task" link below the table is removed at the
same time.

Change-Id: I1c1ebb47a085b980f83fa1630c040d73f510199c
Resolves: #32753
Releases: 4.7, 4.6, 4.5
Reviewed-on: http://review.typo3.org/7473
Reviewed-by: Andreas Wolf
Tested-by: Andreas Wolf