Refactored add child feature, to have all the page specific code in CMS_PAGE_NODE_TYPE_WEBFORM_MANAGER (and not in the general node code).

Note that ideally PAGE could be a separated page node module (as it is for blog).
Added listing of children for each "page".

modules/node/persistence/cms_node_storage_i.e

@@ -117,9 +117,19 @@ feature -- Access
 		deferred
 		end
 
+feature -- Access: outline
+
+	children (a_node: CMS_NODE): detachable LIST [CMS_NODE]
+			-- Children of node `a_node'.
+			-- note: this is the partial version of the nodes.
+		deferred
+		end
+
 	available_parents_for_node (a_node: CMS_NODE): LIST [CMS_NODE]
 			-- Given the node `a_node', return the list of possible parent nodes id
 		deferred
+		ensure
+			a_node_excluded: across Result as ic all not a_node.same_node (ic.item) end
 		end
 
 feature -- Change: Node