Add support for custom duplicate of SceneGraphNode

Source/Editor/SceneGraph/SceneGraphNode.cs

@@ -65,6 +65,11 @@ namespace FlaxEditor.SceneGraph
         /// </summary>
         public virtual bool CanCopyPaste => true;
 
+        /// <summary>
+        /// Gets a value indicating whether this instance can be duplicated by the user.
+        /// </summary>
+        public virtual bool CanDuplicate => true;
+
         /// <summary>
         /// Gets a value indicating whether this node can be deleted by the user.
         /// </summary>
@@ -359,6 +364,17 @@ namespace FlaxEditor.SceneGraph
         {
         }
 
+        /// <summary>
+        /// Duplicates this object. Valid only if <see cref="CanDuplicate"/> returns true.
+        /// </summary>
+        /// <param name="undoAction">The undo action that duplicated the object (already performed), null if skip it.</param>
+        /// <returns>The duplicated object node.</returns>
+        public virtual SceneGraphNode Duplicate(out IUndoAction undoAction)
+        {
+            undoAction = null;
+            return null;
+        }
+
         /// <summary>
         /// Releases the node and the child tree. Disposed all GUI parts and used resources.
         /// </summary>