code: public RadioBoxList<V> addListener(Listener listener) {
          if(listener != null && !listeners.contains(listener)) {
              listeners.add(listener);
          }
          return this;
      }
docs: Adds a new listener to the {@code RadioBoxList} that will be called on certain user actions
  @param listener Listener to attach to this {@code RadioBoxList}
  @return Itself