improve preview of grid questions and sorting

*added likertnode
*group subquestions are in an extra <ul>
*sorting of questions in a group must be handled when editing that group
*remove adminOptions-decorators of questions in a group
*groupText is not a seperate question anymore (data.yml)
*(pre)view helper: form elements are not added to the form at the
pageNode level, but at lower levels. the displaygroup is added at the
group level (prevents wrong sorting when displaying form)