@Override
  public void actionPerformed(ActionEvent e) {
    if (e.getSource() == left) {
      weeknum -= 1;

      gls.notifyListeners(ChangeType.PREVWEEK, null);
    } else if (e.getSource() == right) {
      weeknum += 1;
      gls.notifyListeners(ChangeType.NEXTWEEK, null);
    } else {
      ArrayList<Object> a = new ArrayList<Object>();
      a.add(e.getSource());
      gls.notifyListeners(ChangeType.APPBUTTON, a);
    }
  }
 public void addGuiListener(GUIListener l) {
   gls.add(l);
 }