Creates an item in the tree and adds it as a child of parent.
If parent is null, the root item will be the parent, or the new item will be the root itself if the tree is empty.
The new item will be the idxth child of parent, or it will be the last child if there are not enough siblings.
Creates an item in the tree and adds it as a child of parent. If parent is null, the root item will be the parent, or the new item will be the root itself if the tree is empty. The new item will be the idxth child of parent, or it will be the last child if there are not enough siblings.