added extension.js, trying to add legacy sprites for tree icons, updates tree icon filters (still not working)

src/Umbraco.Core/Serialization/JsonToStringConverter.cs

@@ -0,0 +1,33 @@
+using System;
+using Newtonsoft.Json;
+using Newtonsoft.Json.Linq;
+
+namespace Umbraco.Core.Serialization
+{
+    /// <summary>
+    /// This is used in order to deserialize a json object on a property into a json string since the property's type is 'string'
+    /// </summary>
+    internal class JsonToStringConverter : JsonConverter
+    {
+        public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
+        {
+            throw new NotImplementedException();
+        }
+
+        public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
+        {
+            if (reader.ValueType == typeof(string))
+            {
+                return reader.Value;
+            }
+            // Load JObject from stream
+            JObject jObject = JObject.Load(reader);
+            return jObject.ToString();
+        }
+
+        public override bool CanConvert(Type objectType)
+        {
+            return typeof(string).IsAssignableFrom(objectType);
+        }
+    }
+}