add `track.delete`

[Kyle Stetz]

A few references created automatically in the TimeSingleton prevent tracks from being garbage collected. These references should be removed through a track.delete method.

Things may get a bit complicated with any lingering in schedulers, but hopefully they are easy to dereference.

[Kyle Stetz]

Also, groups need to be notified. This could be done through an event or through the TimeSingleton (if it held references to tracks and groups separately, which it should anyway).

[Kyle Stetz]

There is no way to delete a track from within itself (only by referencing its parent object, e.g. delete window.t), so at least for now deleting a track will have to be a two-step operation.

The track needs to know about its impending destruction so that it can tell TimeSingleton to remove references to it. The API for now could either be:

var t = new track()
t.destroy(), delete window.t

or

var t = new track()
t.delete(), delete window.t

I am leaning toward the former since calling track.delete is misleading if it isn't actually deleted.

[Kyle Stetz]

Closed with dca6752.