Added playlist view and initial playlist view logic.

Harmonia.UI/ViewModels/ViewModelLocator.cs

@@ -12,4 +12,7 @@ public class ViewModelLocator
 
     public static PlayingSongInfoViewModel PlayingSongInfoViewModel
         => App.ServiceProvider.GetRequiredService<PlayingSongInfoViewModel>();
+
+    public static PlaylistViewModel PlaylistViewModel
+        => App.ServiceProvider.GetRequiredService<PlaylistViewModel>();
 }