Implement Promise.completeAsync().

[player-03]

Like #1828, but designed for the Promise class because I think that's more appropriate. Compared to new Future(), completeAsync() 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().

This PR includes #1837, on the assumption that we'll merge that first.

[player-03]

For convenience, here's the new function's documentation and signature:

/**
	Runs the given function asynchronously, and resolves this `Promise` with
	the complete, error, and/or progress events sent by that function.
	Sample usage:
	```haxe
	function examplePromise():Future<String>
	{
		var promise = new Promise<String>();
		promise.completeAsync(function(state:State, output:WorkOutput):Void
			{
				output.sendProgress({progress:state.progress, total:10});
				state.progress++;
				if (state.progress == 10)
				{
					output.sendComplete("Done!");
				}
			},
			{progress: 0}, MULTI_THREADED);
		return promise.future;
	}
	var future = examplePromise();
	future.onComplete(function(message) { trace(message); });
	future.onProgress(function(loaded, total) { trace("Progress: " + loaded + ", " + total); });
	```
	@param doWork A function to perform work asynchronously. For best results,
	see the guidelines in the `ThreadPool` class overview.
	@param state The value to pass to `doWork`.
	@param mode Which mode to run the job in: `SINGLE_THREADED` or `MULTI_THREADED`.
**/
public function completeAsync(doWork:WorkFunction<State->WorkOutput->Void>, ?state:State, ?mode:ThreadMode = MULTI_THREADED):Void