FormGroup should not implement IFormModelUpdateListener

- this interface should only be implemented by form components
  to update their _own_ model
- the wicket Form already visits all its child form components
  and call their updateModel method
  -> the updateModel method of the FormGroup causes that the
     updateModel method of each of its children gets called twice
Merge pull request #416 from romueller/master

FormGroup should not implement IFormModelUpdateListener