feat: move to async POSIX signal handler

Instead of trying to handle signals for every step of the `cursive`
event loop, move the signal handling into its own asynchronous task and
send callbacks to `cursive` when a signal arrives.

Cargo.toml

@@ -66,6 +66,7 @@ wl-clipboard-rs = {version = "0.7", optional = true}
 
 [target.'cfg(unix)'.dependencies]
 signal-hook = "0.3.0"
+signal-hook-tokio = { version = "0.3.1", features = ["futures-v0_3"] }
 
 [dependencies.rspotify]
 default-features = false