[BUGFIX] Don't override the class on the Form elementWrap

This Changeset fixes an issue, which made it impossible
to give the layout.elementWrap a class attribute, because it
is was simple overridden.
Now it checks if a class is set and appends the default csc-form-*
classes instead of overriding it.

Resolves: #48404
Releases: master, 6.2
Change-Id: I20fec83d28b2e727f268ee00dab830c39c5f15a1
Reviewed-on: http://review.typo3.org/35400
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>