[BUGFIX] Workspaces: Properly use slots to modify data

Using slots in GridDataService does not work since passing
arguments by reference does not work. Since TYPO3 CMS 6.2
it's possible that the SignalSlotDispatcher returns
(modified) arguments.

Resolves: #60926
Releases: master, 6.2
Change-Id: If7f3a782bf8870bfb158c566df01cf2311fce048
Reviewed-on: http://review.typo3.org/41191


Reviewed-by: Benjamin Mack <benni@typo3.org>
Tested-by: Benjamin Mack <benni@typo3.org>