Windows: Add AboutWindow

Elwig/Windows/MainWindow.xaml.cs

@@ -59,6 +59,11 @@ namespace Elwig.Windows {
             }
         }
 
+        private void Menu_Help_About_Click(object sender, RoutedEventArgs evt) {
+            var w = new AboutWindow();
+            w.Show();
+        }
+
         private async void Menu_Help_Update_Click(object sender, RoutedEventArgs evt) {
             await App.CheckForUpdates(true);
         }