Add a Avalonia about window default implementation.

src/Avalonia.Dialogs/DefaultAboutWindow.xaml

@@ -0,0 +1,109 @@
+<Window xmlns="https://github.com/avaloniaui"
+        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+        MaxWidth="640"
+        MaxHeight="220"
+        MinWidth="640"
+        MinHeight="220"
+        Title="About Avalonia"
+        Background="Purple"
+        x:Class="Avalonia.Dialogs.DefaultAboutWindow">
+  <Grid>
+    <Grid.Styles>
+      <Style Selector="Rectangle.Abstract">
+        <Setter Property="Fill" Value="White" />
+        <Setter Property="Width" Value="750" />
+        <Setter Property="Height" Value="700" />
+      </Style>
+    </Grid.Styles>
+    <Canvas>
+      <Rectangle Classes="Abstract"
+                 Canvas.Top="90"
+                 Opacity="0.132">
+        <Rectangle.RenderTransform>
+          <RotateTransform Angle="-2" />
+        </Rectangle.RenderTransform>
+      </Rectangle>
+      <Rectangle Classes="Abstract"
+                 Canvas.Top="95"
+                 Opacity="0.3">
+        <Rectangle.RenderTransform>
+          <RotateTransform Angle="-4" />
+        </Rectangle.RenderTransform>
+      </Rectangle>
+      <Rectangle Classes="Abstract"
+                 Canvas.Top="100"
+                 Opacity="0.3">
+        <Rectangle.RenderTransform>
+          <RotateTransform Angle="-8" />
+        </Rectangle.RenderTransform>
+      </Rectangle>
+      <Rectangle Classes="Abstract"
+                 Canvas.Top="105"
+                 Opacity="0.7">
+        <Rectangle.RenderTransform>
+          <RotateTransform Angle="-12" />
+        </Rectangle.RenderTransform>
+      </Rectangle>
+    </Canvas>
+    <Grid x:Name="AvaloniaLogo"
+          HorizontalAlignment="Left"
+          VerticalAlignment="Top">
+      <Grid.RenderTransform>
+        <TransformGroup>
+          <ScaleTransform ScaleX="0.35"
+                          ScaleY="0.35" />
+          <TranslateTransform X="-80"
+                              Y="-65" />
+        </TransformGroup>
+      </Grid.RenderTransform>
+      <Path HorizontalAlignment="Left"
+            VerticalAlignment="Top"
+            Data="m 150.66581 0.66454769 c -54.77764 0 -101.0652 38.86360031 -112.62109 90.33008031 a 26.1 26.1 0 0 1 18.92187 25.070312 26.1 26.1 0 0 1 -18.91992 25.08202 c 11.56024 51.46073 57.8456 90.31837 112.61914 90.31837 63.37832 0 115.40039 -52.02207 115.40039 -115.40039 0 -63.378322 -52.02207 -115.40039231 -115.40039 -115.40039231 z m 0 60.00000031 c 30.95192 0 55.40039 24.44847 55.40039 55.400392 0 30.9519 -24.44847 55.40039 -55.40039 55.40039 -30.95191 0 -55.40039 -24.44848 -55.40039 -55.40039 0 -30.951922 24.44848 -55.400392 55.40039 -55.400392 z">
+
+        <Path.Fill>
+          <LinearGradientBrush StartPoint="272,411"
+                               EndPoint="435,248">
+            <LinearGradientBrush.GradientStops>
+              <GradientStop Color="#B8B8B8"
+                            Offset="0" />
+              <GradientStop Color="#FFFFFF"
+                            Offset="0.6784" />
+            </LinearGradientBrush.GradientStops>
+          </LinearGradientBrush>
+        </Path.Fill>
+      </Path>
+      <Rectangle Name="letterAStem"
+                 HorizontalAlignment="Left"
+                 VerticalAlignment="Top"
+                 Margin="206.06355, 114.56503, 0, 0"
+                 Width="60"
+                 Height="116.2"
+                 Fill="#FFFFFF" />
+      <Ellipse Name="notchDot"
+               HorizontalAlignment="Left"
+               VerticalAlignment="Top"
+               Margin="9.6,95.8,0,0"
+               Width="40.6"
+               Height="40.6"
+               Fill="#BABABA"
+               StrokeThickness="7.77769947"
+               Opacity="1" />
+    </Grid>
+    <StackPanel HorizontalAlignment="Left"
+                VerticalAlignment="Top"
+                Margin="113,15,0,0">
+      <TextBlock Text="Avalonia 0.9"
+                 FontSize="40"
+                 Foreground="White" />
+      <TextBlock Text="Development Build"
+                 Margin="0,-10,0,0"
+                 FontSize="15"
+                 Foreground="White" />
+    </StackPanel>
+    <TextBlock Text="Version String"
+               HorizontalAlignment="Right"
+               VerticalAlignment="Bottom"
+               Margin="5"
+               FontSize="15" />
+  </Grid>
+</Window>

src/Avalonia.Dialogs/DefaultAboutWindow.xaml.cs

@@ -0,0 +1,16 @@
+// Copyright (c) The Avalonia Project. All rights reserved.
+// Licensed under the MIT license. See licence.md file in the project root for full license information.
+
+using Avalonia.Controls; 
+using Avalonia.Markup.Xaml;
+
+namespace Avalonia.Dialogs
+{
+    public class DefaultAboutWindow : Window
+    {
+        public DefaultAboutWindow()
+        {
+            AvaloniaXamlLoader.Load(this);
+        }
+    }
+}