Redesigned hooks system (moving from CMS_RESPONSE to CMS_API).

Indeed, hooks does not require RESPONSE interface,
   and should be setup before, at the system level (i.e CMS_API)
Added exportation solution via CMS_HOOK_EXPORT.
Updated blocks settings for demo example project.

modules/blog/persistence/cms_blog_storage_i.e

@@ -6,8 +6,12 @@ note
 deferred class
 	CMS_BLOG_STORAGE_I
 
-inherit
-	CMS_NODE_STORAGE_I
+feature -- Error Handling
+
+	error_handler: ERROR_HANDLER
+			-- Error handler.
+		deferred
+		end
 
 feature -- Access