LinkWidget call to @@getVocabulary should be based on INavigationRoot

[keul]

See

plone.app.z3cform/plone/app/z3cform/widget.py

Line 714 in 533c9f7

getSite().absolute_url(0)

This call is always mapped to the site root, while in dozen of other places Plone use an approach based to INavigationRoot. IMHO this must be changed (or at least: load the root by using an adapter... or both!)