Improve form-field "file" (and subclasses)
- add clear button for clearning files/images added added in a
prevision revision (unless flag "sticky" is used)
- tie revisions of the FormPage/WorkflowInstance with the revision
of the uploaded file (the content of the formfield is now a
list of attribute value pairs that might contain a revision_id)
- allow multiple uploads of files with the same name (distinguish with
reusing old content)
- caveat: export should strip the revision_id from the content
of the form-field.
