Refactor: separate Spotify API from player logic

The separation is not perfect yet, but it's a start and makes the entire codebase much easier to read.

src/main.rs

@@ -34,6 +34,7 @@ mod serialization;
 mod sharing;
 mod show;
 mod spotify;
+mod spotify_api;
 mod spotify_url;
 mod spotify_worker;
 mod theme;