Highlight active playlist. Various optimizations and fixes.

Harmonia.Tests/AudioPlayerTests.cs

@@ -21,7 +21,7 @@ internal class TestAudioPlayer(IAudioEngine audioEngine, IPlaylistManager playli
         if (Playlist == null || PlayingSong == null)
             return -1;
 
-        return Playlist.Songs.IndexOf(PlayingSong);
+        return Playlist.IndexOf(PlayingSong);
     }
 }
 
@@ -54,10 +54,7 @@ public class AudioPlayerTests
             new Song() { FileName = "Song7.mp3" }
         ];
 
-        Playlist playlist = new()
-        {
-            Name = "Playlist1"
-        };
+        Playlist playlist = new("Playlist1");
 
         playlist.AddSongs(songs);
 
@@ -65,7 +62,7 @@ public class AudioPlayerTests
 
         _playlistManager = Substitute.For<IPlaylistManager>();
         _playlistManager.Playlists.Returns([playlist]);
-        _playlistManager.GetPlaylist(Arg.Any<PlaylistSong>()).Returns(playlist);
+        _playlistManager.FindPlaylistContaining(Arg.Any<PlaylistSong>()).Returns(playlist);
 
         _audioPlayer = new TestAudioPlayer(_audioEngine, _playlistManager);
         _audioPlayer.SetPlaylist(playlist);