[Playlist] schema -> validation

[Lucas Gonze]

The old "schema" page on the web site has been replaced by a new 
"validation" page.  Well, actually, the old page is still there but 
there are no links to it within the site and there is an added link to 
the new page in the site-wide header.

The new page links to:
- Sebastian's web-based validator
- Matthias' new and improved XSD
- Matthias' new and improved RNG