Added playing song info view and view model.

Harmonia.UI/App.axaml.cs

@@ -21,6 +21,7 @@ public partial class App : Application
         services.AddSingleton<MainViewModel>();
         services.AddSingleton<MainWindow>();
         services.AddSingleton<PlaybackBarViewModel>();
+        services.AddSingleton<PlayingSongInfoViewModel>();
 
         services.AddHarmonia();
 
@@ -54,7 +55,7 @@ public partial class App : Application
             //    DataContext = new MainViewModel()
             //};
 
-            singleViewPlatform.MainView = ServiceProvider.GetRequiredService<MainWindow>();
+            singleViewPlatform.MainView = ServiceProvider.GetRequiredService<MainView>();
         }
 
         base.OnFrameworkInitializationCompleted();