form-fields:
- new helper API call: "dict_to_fc" to convert from a dict to a form constraint spec.
- extended formfield answer and word statistics
- ckeditor4: add dimensions (width and height) when available to the configuration
- prefer variables over method calls
- added answer hints (one word, multiple words, ...) to text_fields widget
- form_page: maintain list of references such that the referenced page
knows if and from where it is referenced
