Add project files.

ObservatoryCore/UI/MainApplication.axaml.cs

@@ -0,0 +1,29 @@
+using Avalonia;
+using Avalonia.Controls.ApplicationLifetimes;
+using Avalonia.Markup.Xaml;
+using Observatory.UI.ViewModels;
+
+namespace Observatory.UI
+{
+    public class MainApplication : Application
+    {
+        public override void Initialize()
+        {
+            AvaloniaXamlLoader.Load(this);
+        }
+
+        public override void OnFrameworkInitializationCompleted()
+        {
+            if (ApplicationLifetime is IClassicDesktopStyleApplicationLifetime desktop)
+            {
+                var pluginManager = PluginManagement.PluginManager.GetInstance;
+                desktop.MainWindow = new Views.MainWindow()
+                {
+                    DataContext = new MainWindowViewModel(pluginManager)
+                };
+            }
+
+            base.OnFrameworkInitializationCompleted();
+        }
+    }
+}