Add the property's type "Document" (Nuxeo 7.2+)

> Add a "Document" entry in the type dropdown
> when it's selected, open the advanced field configuration screen
> select the type string
> add a reference section
> add a dropdown reference choice with values None and "Document" and others existing
> automatically select the "Document" value
> add a reference parameter "store" which contains "id" and "path"
> default to "id"
> if the reference usergroup is selected and the type is not string, add an error (id or path are always strings)
> manage the corresponding XSD generation