Add GUI template

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

HyperbolicDownloaderGUI/Views/MainWindow.axaml.cs

@@ -0,0 +1,22 @@
+using Avalonia;
+using Avalonia.Controls;
+using Avalonia.Markup.Xaml;
+
+namespace HyperbolicDownloaderGUI.Views
+{
+    public partial class MainWindow : Window
+    {
+        public MainWindow()
+        {
+            InitializeComponent();
+#if DEBUG
+            this.AttachDevTools();
+#endif
+        }
+
+        private void InitializeComponent()
+        {
+            AvaloniaXamlLoader.Load(this);
+        }
+    }
+}