replace v6 with experimental v7 code

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

@@ -0,0 +1,15 @@
+
+namespace MpvNet.Windows.WPF.MsgBox;
+
+public enum MessageBoxButtonEx { OK = 0, OKCancel, AbortRetryIgnore, YesNoCancel, YesNo, RetryCancel }
+
+public enum MessageBoxResultEx { None = 0, OK, Cancel, Abort, Retry, Ignore, Yes, No }
+
+public enum MessageBoxButtonDefault
+{
+    OK, Cancel, Yes, No, Abort, Retry, Ignore, // specific button
+    Button1, Button2, Button3,                 // button by ordinal left-to-right position
+    MostPositive, LeastPositive,               // button by positivity
+    Forms,                                     // button according to the Windows.Forms standard messagebox
+    None                                       // no default button
+}