`show_async` doesn't compile on MacOS

[tehmatt]

It would be nice if Notification::show_async() worked on macos, even though there's not really anything async to do, since it'll avoid undocumented compilation failures when using show_async.

[hoodie]

If I remember correctly, this was intentional. The macOS send call is blocking so you can't just wrap it into an async function, that would block the runtime. At the very least, you would have to wrap it into an abstraction like blocking. But that would've made a bunch of things even more complicated so I avoided the work for now. I don't know if it's trivially possible to spawn a notification on macOS from another threat in the main threat.
If you want to try this out and tell me that it works, I would be very grateful for your support.