replace v6 with experimental v7 code

src/MpvNet.Windows/WPF/MsgBox/MsgBoxExDelegate.cs

@@ -0,0 +1,16 @@
+
+using System.Windows;
+
+namespace MpvNet.Windows.WPF.MsgBox;
+
+public abstract class MsgBoxExDelegate
+{
+    public string? Message { get; set; }
+    public string? Details { get; set; }
+    public DateTime MessageDate { get; set; }
+
+    public virtual MessageBoxResult PerformAction(string message, string? details = null)
+    {
+        throw new NotImplementedException();
+    }
+}