Add org.apache.wicket.markup.html.form.IChoiceRenderer#getObject( String idValue, IModel<? extends List<? extends T>> choices) method

"This method should return the object that produces idValue when org.apache.wicket.markup.html.form.IChoiceRenderer#getIdValue(T object, int index) is called."

Impact: large, there are around 25 IChoiceRenderer implementations in the code base. Each of them should get this additional method, which involves understanding all the model logic of each implementation.