Use streams instead of callbacks to get the playback state, etc.

[hacker1024]

Fixes #18 .

[ryanheise]

Looks good! I've merged it and will add further changes: encapsulate all state changes, slight naming changes, cancel the stream subscription when disconnecting in the example.

[hacker1024]

cancel the stream subscription when disconnecting in the example

I just made this mistake; be sure to also cancel it in the dispose() method of your stateful widget.

[hacker1024]

It might be good to put this comment from the Flutter docs in your README.

If a State's build method depends on an object that can itself change
state, for example a ChangeNotifier or Stream, or some other object to
which one can subscribe to receive notifications, then the State should
subscribe to that object during initState, unsubscribe from the old
object and subscribe to the new object when it changes in
didUpdateWidget, and then unsubscribe from the object in dispose.

[ryanheise]

Interestingly the dispose() method is not called in the example, but I've added in a call to disconnect in there anyway, just because it did look a bit suspicious having a connect in initState and no disconnect in dispose.

With didUpdateWidget, the way I see it is that it's probably not practical for every plugin that provides streams to repeat the usual warnings about remembering to subscribe and cancel correctly and that anyone intending to use streams should read the streams documentation first. But if it proves to be enough of a stumbling block down the track, I may revisit this.