Adds Data field to MenuItem

Terminal.Gui/Views/Menu.cs

@@ -43,6 +43,12 @@ namespace Terminal.Gui {
 		ustring title;
 
 		ShortcutHelper shortcutHelper;
+		
+		/// <summary>
+		/// Gets or sets arbitrary data for the menu item.
+		/// </summary>
+		/// <remarks>This property is not used internally.</remarks>
+		public object Data { get; set; }
 
 		/// <summary>
 		/// Initializes a new instance of <see cref="MenuItem"/>