org.apache.wicket.util.value.Count: add decrement method

Wicket-Examples (LinkPage) use examples specific "ClickCount"-model class.
Should use org.apache.wicket.util.value.Count class (to promote the use of it).
For being usable for all examples, the Count-class needs this additional method:

/**

• Decreases the count value by one.
  */
  public void decrement() {
  count--;
  }

and a "setCount(int count)" method.