move document handlers to session

[GoogleCodeExporter]

I hope an can attend the Pharo Sprint at Feb 18./19. and repurpose it
to hack on Seaside. One thing I'd like to do is moving the document
handlers from registry to session.

Pros:
 * simple
 * adding a document handler no longer potentially expires a session
 * expiring a session expires all it's document handlers
 * session tracking code becomes much simpler

Cons:
 * #urlXXX methods on WARenderContext only work with sessions (a
fallback could be added in theory)
 * #urlXXX methods on WARenderContext need to become class extensions
of the Seaside-Session package
 * not really clean framework-framework Endlösung

Original issue reported on code.google.com by philippe...@gmail.com on 18 Feb 2012 at 12:25