Made repository methods asynchronous. Simplified playlist repository. Use playlist manager in the app, and left playlist repository to only be used by the playlist manager.

Harmonia.Core/Playlists/IPlaylistManager.cs

@@ -2,10 +2,13 @@
 
 public interface IPlaylistManager
 {
+    IReadOnlyList<Playlist> Playlists { get; }
     Playlist? CurrentPlaylist { get; set; }
 
-    void AddPlaylist();
+    Task InitializeAsync();
+    Task<Playlist> AddPlaylistAsync();
     void RemovePlaylist(Playlist playlist);
+    Playlist? GetPlaylist(PlaylistSong playlistSong);
 
     event EventHandler? CurrentPlaylistChanged;
     event EventHandler<PlaylistAddedEventArgs> PlaylistAdded;