Replace callbacks with streams

[robert-ancell]

Currently you register callbacks to handle signals:

var client = DBusClient.session();
var subscription = await client.subscribeSignals((path, interface, member, values) {
  var count = (values[0] as DBusUint64).value;
  print('Ping ${count}!');
}, sender: 'com.example.DartTest');
...
client.unsubscribeSignals();

I think the more Dart way of doing things is to use a stream, which would give an API like this:

var stream = await client.subscribeSignals(sender: 'com.example.DartTest');
await for (signal in stream) {
  var count = (signal.values[0] as DBusUint64).value;
  print('Ping ${count}!')
}

or with callbacks:

var stream = await client.subscribeSignals(sender: 'com.example.DartTest');
var subscription = stream.listen((signal) {
  var count = (signal.values[0] as DBusUint64).value;
  print('Ping ${count}!')
});
...
subscription.cancel();

[robert-ancell]

The reason the current implementation doesn't use streams is I wasn't experienced enough with Dart at the time to work out how to do it so I just started with callbacks.