This is needed for some premium features where we extend the tracker. Listening to
TrackerSetup event doesn't work there because once this event is triggered, the plugins will call
Piwik.getAsyncTrackers() but on the
TrackerSetup event the tracker has only just been created and not yet added to the list of async trackers. We therefore need to trigger an event when a new tracker is added to the list of async trackers. Those are all internal APIs so no need re developer changelog.