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.

examples/demo/modules/blog/cms_blog_module.e

@@ -35,14 +35,14 @@ feature {CMS_API} -- Module Initialization
 	initialize (api: CMS_API)
 			-- <Precursor>
 		local
-			ct: CMS_BLOG_CONTENT_TYPE
+			ct: CMS_BLOG_NODE_TYPE
 		do
 			Precursor (api)
 
 			if attached {CMS_NODE_API} api.module_api ({NODE_MODULE}) as l_node_api then
 				create ct
 				l_node_api.add_content_type (ct)
-				l_node_api.add_content_type_webform_manager (create {CMS_BLOG_CONTENT_TYPE_WEBFORM_MANAGER}.make (ct))
+				l_node_api.add_content_type_webform_manager (create {CMS_BLOG_NODE_TYPE_WEBFORM_MANAGER}.make (ct))
 					-- Add support for CMS_BLOG, which requires a storage extension to store the optional "tags" value
 					-- For now, we only have extension based on SQL statement.
 				if attached {CMS_NODE_STORAGE_SQL} l_node_api.node_storage as l_sql_node_storage then