fix: block until shutdown

[restalex]

When updating ytermusic I saw a huge spike in cpu usage (from 0.2 % to 100 % CPU core usage).
I used samply for profiling and saw that most of it was because of the loop checking if the shutdown signal have been sent.
I wanted to return to an async function but I did not want to break the macOS run_window_handler and I assume that it is made that way for a reason.
Instead of refactoring everything again, I just placed 100ms sleep into the loop, which I think is a reasonable time for a shutdown procedure.
This is not perfect, but with that we obtain a similar performance than before.

EDIT: I've just noticed that should close issue #132 .

[ccgauche]

I added a wait until the app is killed. This will be faster to react than a wait and even lighter. But on some system the OS may say that the app isn't responding. (If you can check for that it would be perfect).

[restalex]

I've tested (this time by stashing the previous modifications before) and there was a simple error compilation on the if condition. I've rewritten it and tested it. As expected we obtain better results than before. I did not thought of implementing a mutex, thanks for that!

[ccgauche]

Thanks for the contribution!