[BUGFIX] EXT:form - Allow setting classes via layout for elements

Allow to add a class attribute defined in the "layout." section to the
desired element tag. This is only needed if the form is in
compatibility mode.

Resolves: #70209
Releases: master
Change-Id: I63c2ed839218ae206359daac4ff20b95f7e38ded
Reviewed-on: http://review.typo3.org/43742


Reviewed-by: Bjoern Jacob <bjoern.jacob@tritum.de>
Tested-by: Bjoern Jacob <bjoern.jacob@tritum.de>
Reviewed-by: Frans Saris <franssaris@gmail.com>
Tested-by: Frans Saris <franssaris@gmail.com>