add a11y identifiers for windows.

samples/IntegrationTestApp/MainWindow.axaml

@@ -4,6 +4,7 @@
         xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
         mc:Ignorable="d" d:DesignWidth="800" d:DesignHeight="450"
         x:Class="IntegrationTestApp.MainWindow"
+        Name="MainWindow"
         Title="IntegrationTestApp">
   <NativeMenu.Menu>
     <NativeMenu>

samples/IntegrationTestApp/ShowWindowTest.axaml

@@ -1,6 +1,7 @@
 <Window xmlns="https://github.com/avaloniaui"
         xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
         x:Class="IntegrationTestApp.ShowWindowTest"
+        Name="SecondaryWindow"
         Title="Show Window Test">
   <Grid ColumnDefinitions="Auto,Auto" RowDefinitions="Auto,Auto,Auto,Auto,Auto,Auto,Auto">
     <Button Name="CloseWindow" Grid.Row="0" Click="CloseWindow_Click">Close</Button>