Add GUI template

- Add AvaloniaUI GUI template to the project
- Add missing newline to the CLI output

HyperbolicDownloaderGUI/Program.cs

@@ -0,0 +1,28 @@
+using Avalonia;
+using Avalonia.ReactiveUI;
+
+using System;
+
+namespace HyperbolicDownloaderGUI
+{
+    internal static class Program
+    {
+        // Initialization code. Don't use any Avalonia, third-party APIs or any
+        // SynchronizationContext-reliant code before AppMain is called: things aren't initialized
+        // yet and stuff might break.
+        [STAThread]
+        public static void Main(string[] args)
+        {
+            BuildAvaloniaApp().StartWithClassicDesktopLifetime(args);
+        }
+
+        // Avalonia configuration, don't remove; also used by visual designer.
+        public static AppBuilder BuildAvaloniaApp()
+        {
+            return AppBuilder.Configure<App>()
+                           .UsePlatformDetect()
+                           .LogToTrace()
+                           .UseReactiveUI();
+        }
+    }
+}