[BUGFIX] Make SingleTasks toggleable from scheduler list view

The toggle button for tasks does not work for single tasks. This patch
enables the functionality and registers a single task for execution on
next scheduler run when enabled.

Additionally class constants are introduced for the two available types.

Change-Id: I91c7b24bf141ef6735b3fd38e2859d63634d507e
Resolves: #64708
Releases: master, 7.6
Reviewed-on: https://review.typo3.org/46773


Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Daniel Goerz <ervaude@gmail.com>
Tested-by: Daniel Goerz <ervaude@gmail.com>
Tested-by: Riccardo De Contardi <erredeco@gmail.com>
Reviewed-by: Andreas Fernandez <typo3@scripting-base.de>
Tested-by: Andreas Fernandez <typo3@scripting-base.de>
Reviewed-by: Frank Naegler <frank.naegler@typo3.org>
Tested-by: Frank Naegler <frank.naegler@typo3.org>