private void btn_selectExecutor_Click(object sender, RoutedEventArgs e)
        {
            List <User> executors = userContract.GetSubordinates(GlobalSettings.CurrentUser.ID);

            executors.Insert(0, GlobalSettings.CurrentUser);
            SelectUserWindow selectUserWindow = new SelectUserWindow(executors);

            if (selectUserWindow.ShowDialog() == true)
            {
                (this.DataContext as Task).Executor   = selectUserWindow.SelectedUser;
                (this.DataContext as Task).ExecutorID = selectUserWindow.SelectedUser.ID;
            }
        }