[BUGFIX] Fix readable output for type "group" and MM-tables

If a TCA field uses the type "group" and a MM-table, a wrong value is
displayed in the list view. This is due to missing support for MM-tables
for type "group". Only the type "select" has an implementation.

This patch adds the implementation for the group field.

Change-Id: Ia57b7468d493962fc9877f12b685f08eecb012ac
Resolves: #52126
Resolves: #57129
Releases: master, 6.2
Reviewed-on: http://review.typo3.org/23897


Reviewed-by: Xavier Perseguers <xavier@typo3.org>
Tested-by: Xavier Perseguers <xavier@typo3.org>
Reviewed-by: Markus Klein <klein.t3@reelworx.at>
Tested-by: Markus Klein <klein.t3@reelworx.at>