Added AvaloniaUI project.

Harmonia.UI/ViewModels/ViewModelLocator.cs

@@ -0,0 +1,12 @@
+using Microsoft.Extensions.DependencyInjection;
+
+namespace Harmonia.UI.ViewModels;
+
+public class ViewModelLocator
+{
+    public static MainViewModel MainViewModel
+        => App.ServiceProvider.GetRequiredService<MainViewModel>();
+
+    public static PlaybackBarViewModel PlaybackBarViewModel
+        => App.ServiceProvider.GetRequiredService<PlaybackBarViewModel>();
+}