No way to extract a child node from the tree

[alpinistbg]

Previously I have used something like

Node.Parent.Delete(Node.Name);

to extract a Node and use it after the tree was freed. After #23 it is not longer possible.
Perhaps a new method must be provided, like fpjson.TJSONObject.Extract.

See https://forum.lazarus.freepascal.org/index.php/topic,64198.0.html