Added AvaloniaUI project.

Harmonia.UI/App.axaml.cs

@@ -0,0 +1,62 @@
+using Avalonia;
+using Avalonia.Controls.ApplicationLifetimes;
+using Avalonia.Data.Core.Plugins;
+using Avalonia.Markup.Xaml;
+using Harmonia.Core.Extensions;
+using Harmonia.UI.ViewModels;
+using Harmonia.UI.Views;
+using Microsoft.Extensions.DependencyInjection;
+using System;
+
+namespace Harmonia.UI;
+
+public partial class App : Application
+{
+    public static IServiceProvider ServiceProvider { get; private set; }
+
+    static App()
+    {
+        ServiceCollection services = new();
+
+        services.AddSingleton<MainViewModel>();
+        services.AddSingleton<MainWindow>();
+        services.AddSingleton<PlaybackBarViewModel>();
+
+        services.AddHarmonia();
+
+        ServiceProvider = services.BuildServiceProvider();
+    }
+
+    public override void Initialize()
+    {
+        AvaloniaXamlLoader.Load(this);
+    }
+
+    public override void OnFrameworkInitializationCompleted()
+    {
+        // Line below is needed to remove Avalonia data validation.
+        // Without this line you will get duplicate validations from both Avalonia and CT
+        BindingPlugins.DataValidators.RemoveAt(0);
+
+        if (ApplicationLifetime is IClassicDesktopStyleApplicationLifetime desktop)
+        {
+            //desktop.MainWindow = new MainWindow
+            //{
+            //    DataContext = new MainViewModel()
+            //};
+
+            desktop.MainWindow = ServiceProvider.GetRequiredService<MainWindow>();
+        }
+        else if (ApplicationLifetime is ISingleViewApplicationLifetime singleViewPlatform)
+        {
+            //singleViewPlatform.MainView = new MainView
+            //{
+            //    DataContext = new MainViewModel()
+            //};
+
+            singleViewPlatform.MainView = ServiceProvider.GetRequiredService<MainWindow>();
+        }
+
+        base.OnFrameworkInitializationCompleted();
+    }
+}