Fixes tree service from showing that there is children when there isn't, fixes container node issues:

U4-3540 When creating a Container type node - the list view shows all children of the item it's being created underneath
U4-3541 media tree is not catering for container types
Adds container style

src/Umbraco.Web/Trees/ContentTreeControllerBase.cs

@@ -83,6 +83,14 @@ namespace Umbraco.Web.Trees
                 return nodes;
             }
 
+            //before we get the children we need to see if this is a container node
+            var current = Services.EntityService.Get(int.Parse(id), UmbracoObjectTypes.Document);
+            if (current != null && current.AdditionalData.ContainsKey("IsContainer") && current.AdditionalData["IsContainer"] is bool && (bool)current.AdditionalData["IsContainer"])
+            {
+                //no children!
+                return new TreeNodeCollection();
+            }
+
             return PerformGetTreeNodes(id, queryStrings);
         }