A message that posts after a game controller becomes the most recently
used controller.
This is a good time to swap out UI to match the new current controller,
and unregister any handlers with the old current controller.
Use the `.didBecomeCurrent` identifier with `NotificationCenter` to listen
for this message.
```
let observation = NotificationCenter.default.addObserver(of: GCController.self, for: .didBecomeCurrent) { message in
let controller = message.controller
}
```
Converts a posted notification into this main actor message type for any observers.
To implement this method in your own `MainActorMessage` conformance, retrieve values from the ``Notification``'s ``Notification/userInfo`` and set them as properties on the message.
- Parameter notification: The posted ``Notification``.
- Returns: The converted `MainActorMessage` or `nil` if conversion is not possible.