private void ButtonInsertCoin_Click(object sender, RoutedEventArgs e)
 {
     if (ListBoxCoins.SelectedIndex < 0)
     {
         TextBoxTimeUntil.Text = "Bitte Münzen einwerfen!";
     }
     else
     {
         FastClock.Instance.IsRunning = false;
         int[] coins = { 10, 20, 50, 100, 200 };
         int   coin  = coins[ListBoxCoins.SelectedIndex];
         _parkingTime          = _slotMachine.Insert(coin);
         TextBoxTimeUntil.Text = _parkingTime;
     }
 }