[BUGFIX] Cannot use slot to modify grid data in workspace module

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