Added application session services, along with initial restore and tracker implementations.

Harmonia.Core/Playlists/IPlaylistManager.cs

@@ -9,6 +9,7 @@ public interface IPlaylistManager
     Task<Playlist> AddPlaylistAsync();
     void RemovePlaylist(Playlist playlist);
     Playlist? FindPlaylistContaining(PlaylistSong playlistSong);
+    Playlist? FindPlaylistContaining(string playlistSongUID);
 
     event EventHandler? CurrentPlaylistChanged;
     event EventHandler<PlaylistAddedEventArgs> PlaylistAdded;