Add GUI template

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

HyperbolicDownloaderGUI/Views/MainWindow.axaml

@@ -0,0 +1,17 @@
+<Window xmlns="https://github.com/avaloniaui"
+        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+        xmlns:vm="using:HyperbolicDownloaderGUI.ViewModels"
+        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
+        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
+        mc:Ignorable="d" d:DesignWidth="800" d:DesignHeight="450"
+        x:Class="HyperbolicDownloaderGUI.Views.MainWindow"
+        Icon="/Assets/avalonia-logo.ico"
+        Title="HyperbolicDownloaderGUI">
+
+    <Design.DataContext>
+        <vm:MainWindowViewModel/>
+    </Design.DataContext>
+
+    <TextBlock Text="{Binding Greeting}" HorizontalAlignment="Center" VerticalAlignment="Center"/>
+
+</Window>