Adding mapping Sitemap -> Link -> Page to deal with Content-Language

Adding mapping Sitemap -> Link -> Page to deal with Content-Language
and other representation of a page at the same link.

Tree does not have an iterator, hence the link set will not currently work.
The Page object will do conversion, and templating, preparing menues
as all that work is language dependant. Link matches sitemap.txt entries
and matches the Directory object.