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/persistence/cms_storage_null.e

@@ -68,20 +68,6 @@ feature -- Access: user
 		do
 		end
 
-feature -- User Nodes
-
-	user_collaborator_nodes (a_id: like {CMS_USER}.id): LIST [CMS_NODE]
-			-- Possible list of nodes where the user identified by `a_id', is a collaborator.
-		do
-			create {ARRAYED_LIST [CMS_NODE]} Result.make (0)
-		end
-
-	user_author_nodes (a_id: like {CMS_USER}.id): LIST [CMS_NODE]
-			-- Possible list of nodes where the user identified by `a_id', is the author.
-		do
-			create {ARRAYED_LIST [CMS_NODE]} Result.make (0)
-		end
-
 feature -- Change: user
 
 	new_user (a_user: CMS_USER)