feat: implement overlay messagebox, unify messagebox and dialog layout.

demo/Ursa.Demo/Pages/MessageBoxDemo.axaml

@@ -20,6 +20,7 @@
         <ComboBox ItemsSource="{Binding Icons}" SelectedItem="{Binding SelectedIcon}" />
         <ToggleSwitch Content="Try Long Message" IsChecked="{Binding UseLong}"></ToggleSwitch>
         <ToggleSwitch Content="Show Title" IsChecked="{Binding UseTitle}"></ToggleSwitch>
+        <ToggleSwitch Content="Overlay" IsChecked="{Binding UseOverlay}"></ToggleSwitch>
         <Button Command="{Binding DefaultMessageBoxCommand}" Content="Default" />
         <Button Command="{Binding OkCommand}" Content="OK" />
         <Button Command="{Binding OkCancelCommand}" Content="OKCancel" />