Formhelper.field_class attribute does not get applied to the floating field wrapper

Previously, the field_class attribute could be used with Bootstrap templates to edit the wrapper on each form item in one line. The floating_field.html template is completely missing logic to parse this attribute, while it is still present in other templates. This makes the application of classes on wrappers much more tedious.

I will do a pull request to fix this if I have the time.