Allow coroutines on add_listener.

The current code that process a notification uses the method `call_soon` which expects a callable, why not to use `create_task` (or `ensure_future` for compatibility reasons) after checking if the callback is a coroutine?

There are different ways to do this, either just checking if the callback is a coroutine as described in [here](https://stackoverflow.com/questions/36076619/test-if-function-or-method-is-normal-or-asynchronous) for example. Another way is just to add an extra argument or a different method to register coroutines.

Would it make sense to add a context? I am currently passing extra arguments to the callback via `functools.partial`.

Let me know if something like this would be accepted I can create a PR with the implementation.

Start at the add_listener entry point and the notification processing that currently calls call_soon. Compare create_task and ensure_future for coroutine callbacks, including the existing functools.partial usage for extra arguments. Done should include an agreed callback API and compatibility behavior, with tests covering regular and coroutine listeners.

Written by the indexing model from the issue text.