[FOLLOWUP][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.

Change-Id: I6a5e123bfc0d44d484e5b432e65a4d6cb50b75fb
Resolves: #60926
Releases: 6.2
Reviewed-on: http://review.typo3.org/41209


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