private void ComboBoxItem_MouseDoubleClick(object sender, MouseButtonEventArgs e)
        {
            MarkModel  data   = (MarkModel)((ComboBoxItem)sender).DataContext;
            MarkDialog dialog = new MarkDialog(data);

            dialog.ShowDialog();
        }
        private void AddMarkButton_Clicked(object sender, RoutedEventArgs e)
        {
            Contact    data   = (Contact)((Button)sender).DataContext;
            MarkDialog dialog = new MarkDialog(new MarkModel());

            if (dialog.ShowDialog() == true)
            {
                data.Marks.Add(dialog.MarkClass);
            }
        }