Feature/INavigationRoot

- Add ``INavigationRoot`` interface and ``TemplateAPI.navigation_root``
  property.  The latter returns the first content node in the lineage that
  implements ``INavigationRoot`` or the root node if ``INavigationRoot`` is not
  implemented by any node in the lineage.  Make the ``nav.pt`` template use
  ``api.navigation_root`` instead of ``api.root``.  This allows third party
  add-ons to define content types that can reside somewhere in the content tree
  while still being the root for the navigation.

- Move navigation related view code to new module ``kotti.views.navigation``.
  Deprecate imports from the old locations.

- Remove *some* code that has been deprecated in 0.6 or 0.7.