code: public void addEntityChangeHandler(String entityId, ValueChangeHandler<CmsEntity> handler) {
  
          CmsEntity entity = m_entityBackend.getEntity(entityId);
          if (entity != null) {
              entity.addValueChangeHandler(handler);
          }
      }
docs: Adds the value change handler to the entity with the given id.<p>
  
  @param entityId the entity id
  @param handler the change handler