Since we're storing more and more in `JobData`, and the background thread only needs three bits of that data, I added those to `ThreadEvent` so we don't have to pass the full object. This may improve performance in HTML5 specifically, where passing a class instance incurs an overhead.

Compared to `new Future()`, this offers:

- The option to run on the main thread.
- The option to send progress events.
- The ability to interrupt the job by calling `promise.error()`.