Add support for custom drag&drop into level/prefab viewports from custom asset items

Source/Editor/Content/Items/PrefabItem.cs

@@ -21,6 +21,18 @@ namespace FlaxEditor.Content
         {
         }
 
+        /// <inheritdoc />
+        public override bool OnEditorDrag(object context)
+        {
+            return true;
+        }
+
+        /// <inheritdoc />
+        public override Actor OnEditorDrop(object context)
+        {
+            return PrefabManager.SpawnPrefab(FlaxEngine.Content.LoadAsync<Prefab>(ID), null);
+        }
+
         /// <inheritdoc />
         public override ContentItemType ItemType => ContentItemType.Asset;