Enables listview children in dialog trees

By default trees doesnt show children of listview nodes, this enables
them in dialog trees, otherwise content pickers cannot select childrens
of a listview node

src/Umbraco.Web/Trees/ContentTreeController.cs

@@ -98,7 +98,8 @@ namespace Umbraco.Web.Trees
             {
 
                 //Special check to see if it ia a container, if so then we'll hide children.
-                var isContainer = e.IsContainer();
+                //we only perform this check for non-dialog trees since pickers needs access to these nodes
+                var isContainer = e.IsContainer() && (queryStrings.Get("isDialog") != "true");
 
                 var node = CreateTreeNode(
                     e.Id.ToInvariantString(),