Updating to objc2 and adding async support

[dscso]

I have been writing a version of this crate but with interaction support based on the NSUserNotification Library.
Here is a working example:
https://github.com/dscso/mac-notifications

let mut provider = NotificationProvider::new("Terminal");

// callback for notification interaction
provider.set_callback(|id, resp| {
   println!("Notification {} clicked: {:?}", id, resp);
});

// sends notification
Notification::new()
    .title("Hello")
    .reply(true)
    .send().unwrap();

// run the main loop. If some notification gets interacted with, the callback will be called
for _ in 0..50 {
    provider.run_main_loop_once();
}

I've improved

• memory leaks (now there should be none)
• expose delegate (user of the library can set a new delegate)
• give control over main_loop (like in Version 2 #51)
• moved to obj2 since it provides a much more stable/easy to use API
• moved most of the stuff I could form the .m file to Rust 🦀

What has to be done:

• action buttons not working yet
• testing for bugs/memory bugs

Maybe this could be combined with #51 to create a full 2.0?

[hoodie]

Sounds awesome! Thanks

[dscso]

For the integration of the UNNotification version I don't have a key to sign applications and therefore test it. So I can't help with this. But I can work on a PR for the NSNotificaiton

[hoodie]

just for reference: my friend @payload did a tiny example using a self-signed key that should be good enough for local testing
https://github.com/payload/tiny-usernotifications-example