title: Wicket countdown timer will not self-update
body: <p>I would like to implement a countdown timer in Wicket. I have a clock class:</p>
  
  <pre><code>public class Clock extends Label{
      private static int time;
      public Clock(int mytime,String id,String message){
      super(id,message);
          time=mytime;
          this.setDefaultModelObject(message);
      }
  
      public int getTimeLeft(){
          return time;
      }
  
      public void decrement(){
          time--;
      }
  }
  </code></pre>
  
  <p>and here I attempt to update it every second:</p>
  
  <pre><code>clock.add(new AjaxSelfUpdatingTimerBehavior(Duration.seconds(1)){
      protected void onTimer(AjaxRequestTarget target){
          clock.decrement();
          target.addComponent(clock);
      }           
  });
  </code></pre>
  
  <p>This doesn't work because <code>onTimer</code> is a <code>final</code> method and therefore cannot be overridden. What is the correct approach?</p>