Implemented view node by content type (no more hardcoded cases).

Added CMS_NODE_TYPE as descendant of CMS_CONTENT_TYPE,
  in case we have content which is not a node in the future.
  (probably useless, but for now, this extra abstraction is harmful)
Moved all node related code under node module cluster.
Applied comments from Javier Velilla.
Code cleaning.

src/kernel/cms_common_api.e

@@ -29,21 +29,11 @@ feature -- Access
 			Result := link (u.name, "/user/" + u.id.out, Void)
 		end
 
-	node_link (n: CMS_NODE): like link
-		do
-			Result := link (n.title, "/node/" + n.id.out, Void)
-		end
-
 	user_url (u: CMS_USER): like url
 		do
 			Result := url ("/user/" + u.id.out, Void)
 		end
 
-	node_url (n: CMS_NODE): like url
-		do
-			Result := url ("/node/" + n.id.out, Void)
-		end
-
 feature -- Helper
 
 	is_empty (s: detachable READABLE_STRING_GENERAL): BOOLEAN