Add being able to add a thumbnail to a SpawnableJsonProxy

Source/Editor/Content/Proxy/JsonAssetProxy.cs

@@ -166,6 +166,18 @@ namespace FlaxEditor.Content
         /// <inheritdoc />
         public override string Name { get; } = Utilities.Utils.GetPropertyNameUI(typeof(T).Name);
 
+        private SpriteHandle _thumbnail;
+
+        public SpawnableJsonAssetProxy()
+        {
+            _thumbnail = SpriteHandle.Invalid;
+        }
+        
+        public SpawnableJsonAssetProxy(SpriteHandle thumbnail)
+        {
+            _thumbnail = thumbnail;
+        }
+
         /// <inheritdoc />
         public override bool CanCreate(ContentFolder targetLocation)
         {
@@ -177,6 +189,12 @@ namespace FlaxEditor.Content
         {
             Editor.SaveJsonAsset(outputPath, new T());
         }
+        
+        /// <inheritdoc />
+        public override AssetItem ConstructItem(string path, string typeName, ref Guid id)
+        {
+            return _thumbnail.IsValid ? new JsonAssetItem(path, id, typeName, _thumbnail) : base.ConstructItem(path, typeName, ref id);
+        }
 
         /// <inheritdoc />
         public override string TypeName { get; } = typeof(T).FullName;