feat: add icon, improve demo.

demo/Ursa.Demo/Pages/MessageBoxDemo.axaml

@@ -1,17 +1,32 @@
-<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"
-             xmlns:vm="using:Ursa.Demo.ViewModels"
-             x:DataType="vm:MessageBoxDemoViewModel"
-             x:CompileBindings="True"
-             x:Class="Ursa.Demo.Pages.MessageBoxDemo">
+<UserControl
+    x:Class="Ursa.Demo.Pages.MessageBoxDemo"
+    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"
+    xmlns:vm="using:Ursa.Demo.ViewModels"
+    d:DesignHeight="450"
+    d:DesignWidth="800"
+    x:CompileBindings="True"
+    x:DataType="vm:MessageBoxDemoViewModel"
+    mc:Ignorable="d">
+    <UserControl.Styles>
+        <Style Selector="Button">
+            <Setter Property="HorizontalAlignment" Value="Left" />
+            <Setter Property="Margin" Value="8"></Setter>
+        </Style>
+    </UserControl.Styles>
     <StackPanel HorizontalAlignment="Left">
-        <Button Content="Default" Command="{Binding DefaultMessageBoxCommand}"></Button>
-        <Button Content="OK" Command="{Binding OkCommand}" ></Button>
-        <Button Content="OKCancel" Command="{Binding OkCancelCommand}" ></Button>
-        <Button Content="YesNo" Command="{Binding YesNoCommand}" ></Button>
-        <Button Content="YesNoCancel" Command="{Binding YesNoCancelCommand}" ></Button>
+        <ComboBox ItemsSource="{Binding Icons}" SelectedItem="{Binding SelectedIcon}" />
+        <Button Command="{Binding DefaultMessageBoxCommand}" Content="Default" />
+        <Button Command="{Binding OkCommand}" Content="OK" />
+        <Button Command="{Binding OkCancelCommand}" Content="OKCancel" />
+        <Button Command="{Binding YesNoCommand}" Content="YesNo" />
+        <Button Command="{Binding YesNoCancelCommand}" Content="YesNoCancel" />
+        
+        <TextBlock>
+            <Run Text="Last Clicked Result: "></Run>
+            <Run Text="{Binding Result}"></Run>
+        </TextBlock>
     </StackPanel>
 </UserControl>