[BUGFIX] Add gridelements fields to showitems of DCEs

Gridelements requires two fields in tt_content TCA to work without bugs. These fields has not been added to DCE's showitems in TCA yet. Now a viewhelper checks if gridelements is installed and adds the required fields to showitems (in extended tab).

Resolves: #59927
Change-Id: I9d63012c4d8a99cbd2a054e1619a7dad5c7f13c1
Reviewed-on: https://review.typo3.org/31166
Reviewed-by: Armin Ruediger Vieweg
Tested-by: Armin Ruediger Vieweg