Resource Manager: Add support for flexible async notifications.

The ma_async_notification object is used for notifying the application
that an asynchronous operation has completed.

Custom notifications can be implemented by implementing the callback in
ma_async_notification_callbacks. There is currently only a single
callback called onSignal which is fired when the operation completes. A
helper notification which wraps around an ma_event object called
ma_async_notification_event is implemented which you can use as an
example for building your own notifications.