function addHeaderEvents(el) {
    if (el) {
      el.tabIndex = 0;
      goog.a11y.aria.setRole(el, self.getAriaRole());
      goog.dom.classlist.add(el, goog.getCssName('goog-zippy-header'));
      self.enableMouseEventsHandling_(el);
      self.enableKeyboardEventsHandling_(el);
    }
  }
A mouse events handler. If there are two headers it is shared for both.
@type {goog.events.EventHandler<!goog.ui.Zippy>}
@private