Correctly update position.

`Window.Position` isn't an Avalonia property so doesn't auto-update, we need to listen for `PositionChanged`.

samples/IntegrationTestApp/ShowWindowTest.axaml

@@ -13,8 +13,7 @@
              Text="{Binding FrameSize, Mode=OneWay}"/>
 
     <Label Grid.Column="0" Grid.Row="3">Position</Label>
-    <TextBox Name="Position" Grid.Column="1" Grid.Row="3" IsReadOnly="True"
+    <TextBox Name="Position" Grid.Column="1" Grid.Row="3" IsReadOnly="True"/>
-             Text="{Binding Position, Mode=OneWay}"/>
 
     <Label Grid.Column="0" Grid.Row="4">Owner Rect</Label>
     <TextBox Name="OwnerRect" Grid.Column="1" Grid.Row="4" IsReadOnly="True"/>

samples/IntegrationTestApp/ShowWindowTest.axaml.cs

@@ -12,6 +12,7 @@ namespace IntegrationTestApp
         {
             InitializeComponent();
             DataContext = this;
+            PositionChanged += (s, e) => this.GetControl<TextBox>("Position").Text = $"{Position}";
         }
 
         private void InitializeComponent()
@@ -22,6 +23,7 @@ namespace IntegrationTestApp
         protected override void OnOpened(EventArgs e)
         {
             base.OnOpened(e);
+            this.GetControl<TextBox>("Position").Text = $"{Position}";
             this.GetControl<TextBox>("ScreenRect").Text = $"{Screens.ScreenFromVisual(this)?.WorkingArea}";
             this.GetControl<TextBox>("Scaling").Text = $"{PlatformImpl?.DesktopScaling}";