Fixed PlaylistDetailViewModels's "Playlist" object not properly synchronizing (events, locking) to the playlist manager's current playlist at constructor time.

Harmonia.Core/Playlists/PlaylistChangedEventArgs.cs

@@ -0,0 +1,7 @@
+namespace Harmonia.Core.Playlists;
+
+public class PlaylistChangedEventArgs(Playlist? oldPlaylist, Playlist? newPlaylist) : EventArgs
+{
+    public readonly Playlist? OldPlaylist = oldPlaylist;
+    public readonly Playlist? NewPlaylist = newPlaylist;
+}