Add "trackIdle" and "allTracksIdle" callbacks

[danvk]

It would be nice to get some kind of notification when a track has finished loading, regardless of whether it's the initial load or triggered by a call to browser.setLocation.

[dasmoth]

Good idea.

Is there a specific use case you've got in mind for the single-track version?

The general case that comes to mind is an event containing a map of track->fetching-status which fires whenever some statuses have changed -- would that work for you?

[danvk]

Yes, that would work for me.

The use case for an individual track version would be if I wanted to log performance stats on individual loads, or re-style some elements when a track finishes loading.