Sample improves

* UserControl sample.
* Multi-theme support.
* NativeMenu on macOS.
* GitHub workflow for Windows, Linux and MacOS artifact compilation.

src/Controls/MainControl.axaml

@@ -0,0 +1,9 @@
+<UserControl xmlns="https://github.com/avaloniaui" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" 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="AvaloniaCoreRTDemo.Controls.MainControl">
+	<Grid ColumnDefinitions="*,*" RowDefinitions="Auto,Auto,*" Margin="32">
+		<TextBlock Grid.Row="0" Grid.Column="0" Grid.ColumnSpan="2" TextAlignment="Center" Margin="0,6">Welcome to Avalonia UI + NativeAOT!</TextBlock>
+		<Image Grid.Row="1" Grid.Column="0" Stretch="None" Source="{Binding DotNetImage}"/>
+		<Image Grid.Row="1" Grid.Column="1" Stretch="None" Source="{Binding AvaloniaImage}"/>
+		<TextBox Grid.Row="2" Grid.Column="0" Grid.ColumnSpan="2" Watermark="Type here" AcceptsReturn="True" TextWrapping="Wrap"/>
+	</Grid>
+</UserControl>

src/Controls/MainControl.axaml.cs

@@ -0,0 +1,21 @@
+using Avalonia.Controls;
+using Avalonia.Markup.Xaml;
+
+using AvaloniaCoreRTDemo.Controls.ViewModels;
+
+namespace AvaloniaCoreRTDemo.Controls
+{
+    public sealed partial class MainControl : UserControl
+    {
+        public MainControl()
+        {
+            InitializeComponent();
+        }
+
+        private void InitializeComponent()
+        {
+            AvaloniaXamlLoader.Load(this);
+            this.DataContext = new MainViewModel();
+        }
+    }
+}

src/Controls/ViewModels/MainViewModel.cs

@@ -0,0 +1,34 @@
+using System;
+using System.IO;
+using System.Runtime.CompilerServices;
+
+using Avalonia.Media.Imaging;
+
+using ReactiveUI;
+
+namespace AvaloniaCoreRTDemo.Controls.ViewModels
+{
+    internal sealed class MainViewModel : ReactiveObject
+    {
+        private readonly IBitmap _dotNetImage;
+        private readonly IBitmap _avaloniaImage;
+
+        public IBitmap DotNetImage
+        {
+            get { return _dotNetImage; }
+            set { this.RaiseAndSetIfChanged(ref Unsafe.AsRef(this._dotNetImage), value); }
+        }
+
+        public IBitmap AvaloniaImage
+        {
+            get { return _avaloniaImage; }
+            set { this.RaiseAndSetIfChanged(ref Unsafe.AsRef(this._avaloniaImage), value); }
+        }
+
+        public MainViewModel()
+        {
+            this._dotNetImage = new Bitmap(Path.Combine(AppDomain.CurrentDomain.BaseDirectory, "dotnet.png"));
+            this._avaloniaImage = new Bitmap(Path.Combine(AppDomain.CurrentDomain.BaseDirectory, "avalonia.png"));
+        }
+    }
+}