External analytics support: listen for a 'refreshless:load' event

This is probably an issue for google analytics module. Implementers should have a look at

1. https://developers.google.com/analytics/devguides/collection/analyticsjs...
2. https://www.koozai.com/blog/analytics/the-complete-google-analytics-even...
3. https://mjau-mjau.com/blog/ajax-universal-analytics/

We'd want to trigger an event that JS can listen to. Then it's up to the JS that is the glue between a particular analytics service and Drupal to also trigger that analytics service when a page is loaded using RefreshLess.

#2695777: Announce new content to screenreader users when navigating using RefreshLess added a refreshless:load event. Which means this is effectively a duplicate… or rather, a support request.

In the case of Google Analytics, that means they have to do something like this:

window.addEventListener('refreshless:load', function() {
  ga('set', {page: window.location.toString(), title: document.title});
  ga('send', 'pageview');
});