Added playlists view. Added messaging service. Added logic for adding and deleting playlists.

Harmonia.WinUI/ViewModels/ViewModelLocator.cs

@@ -13,6 +13,9 @@ public class ViewModelLocator
     public static PlayingSongViewModel PlayingSongViewModel
         => App.ServiceProvider.GetRequiredService<PlayingSongViewModel>();
 
+    public static PlaylistsViewModel PlaylistsViewModel
+        => App.ServiceProvider.GetRequiredService<PlaylistsViewModel>();
+
     public static PlaylistDetailViewModel PlaylistDetailViewModel
         => App.ServiceProvider.GetRequiredService<PlaylistDetailViewModel>();
 }