Simplified the add child mechanism, by using a query parameter ?parent=nid

(instead of specific node/{nid}/add_child/page url)
Fixed implementation of `CMS_NODE_API.is_node_a_parent_of (..)',
  and improved parent validity checking against cycle.

modules/node/handler/node_view_response.e

@@ -61,7 +61,7 @@ feature -- Execution
 		do
 			l_node := node
 			if l_node = Void then
-				nid := node_id_path_parameter (request)
+				nid := node_id_path_parameter
 				if nid > 0 then
 					l_node := node_api.node (nid)
 				end