[BUGFIX] New form wizard not loading in IE8

Internet Explorer 8 raises an error on JavaScript objects with reserved
words as property names. In the new form wizard an object was created
with class as property name which is a reserved word. Surrounding with
quotes fixes the bug.

Change-Id: Id9ceef79e8503886cdaebbf14ddb4e92c4b7dcea
Fixes: #33504
Releases: 6.0, 4.7, 4.6
Reviewed-on: http://review.typo3.org/8776
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Mario Rimann
Tested-by: Mario Rimann
Reviewed-by: Stefan Neufeind
Reviewed-by: Dmitry Dulepov
Tested-by: Dmitry Dulepov
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn