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.e

@@ -24,7 +24,7 @@ feature {NONE} -- Initialization
 			Precursor
 		end
 
-feature {CMS_CONTENT_TYPE} -- Conversion
+feature -- Conversion
 
 	import_node (a_node: CMS_NODE)
 			-- <Precursor>
@@ -39,7 +39,7 @@ feature -- Access
 
 	content_type: READABLE_STRING_8
 		once
-			Result := {CMS_BLOG_CONTENT_TYPE}.name
+			Result := {CMS_BLOG_NODE_TYPE}.name
 		end
 
 feature -- Access: content