Changes the CreateTreeNode to ensure that a parentId is always passed in - now all tree node models have a parentId property based on the server side data. Changes the JS tree node model to not have a 'parent' property and instead changes this to a parent() method - this allows us to serialize the tree model because otherwise it has cyclical references.

src/Umbraco.Web/Trees/ContentTreeController.cs

@@ -75,6 +75,7 @@ namespace Umbraco.Web.Trees
 
                     var node = CreateTreeNode(
                         e.Id.ToInvariantString(),
+                        id,
                         queryStrings,
                         e.Name,
                         e.ContentTypeIcon,