[PowerAccent] Fix toolbar positioning on first launch (#20135)

As position is being set according to toolbar width&height (depends on num of
characters), on first toolbar-show size of the toolbar is wrong (list items are not set yet).
Therefore, first set characters and then position.

src/modules/poweraccent/PowerAccent.UI/Selector.xaml.cs

@@ -38,8 +38,8 @@ public partial class Selector : Window, IDisposable
         this.Visibility = isActive ? Visibility.Visible : Visibility.Collapsed;
         if (isActive)
         {
-            CenterWindow();
             characters.ItemsSource = chars;
+            CenterWindow();
             Microsoft.PowerToys.Telemetry.PowerToysTelemetry.Log.WriteEvent(new PowerAccent.Core.Telemetry.PowerAccentShowAccentMenuEvent());
         }
     }