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

> Add a Directory 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 Directory and others existing
> automatically select the Directory value
> add a reference parameter "Directory Name" (dropdown with all existing directories)
> default to none
> if no directory is selected and the reference is selected, add an error message on save
> if the reference directory is selected and the type is not string, add an error (directory id are always strings)
> manage the corresponding XSD generation