replace v6 with experimental v7 code

src/MpvNet.Windows/UI/TreeNode.cs

@@ -0,0 +1,11 @@
+
+namespace MpvNet.Windows.UI;
+
+public class TreeNode
+{
+    readonly List<TreeNode> _children = new List<TreeNode>();
+
+    public IList<TreeNode> Children => _children;
+
+    public string Name { get; set; } = "";
+}