Add ability to respond to notifications on macOS

Implement the UNTextInputNotificationAction and UNNotificationCategory stuff so that messages can actually be sent from the notification banner.

This requires adding a UNUserNotificationCenterDelegate object to respond to notification actions. This needs to be registered when the app launches (rather than when we receive a notification), so I have updated main.cpp to incorporate this. If there's a better way... let me know.

I also now properly check for permission access before trying to send a notification. I'm pretty sure it ends up as a no-op if you don't have permission, rather than a hard error, but I'd rather not waste cycles constructing a notification that will never see the light of day if it's not necessary!

See: https://developer.apple.com/documentation/usernotifications/declaring_your_actionable_notification_types?language=objc