Add API to indicate when properties change or devices are added/removed.

[robert-ancell]

The objects detect this, but it's not exposed over the API.

[robert-ancell]

Something like:

var client = UPowerClient();
client.connect();
await for (var name in client.displayDevice.propertyChanges) {
  if (name == 'State') {
    print (client.displayDevice.state);
  }
}