improved flexibility of form-fields

- added "form_field" class to access every field. Jet it is only possible via ".formbody > div" witch is not very precise. Also now you can create a nice gird layout in the form-generator. Wrap fields inside a fieldset, add a "row" class and give every field a class of e.g. "col-3, col-4, col6…"
- wrapped fieldset in "div" to be more consistent