Set PointerOverRoot.

Fixes DevTools not working when pressing Ctrl+Shift (there's a check that `vm.PointerOverRoot` is not null in `MainWindow.xaml.cs`).

src/Avalonia.Diagnostics/Diagnostics/ViewModels/MainViewModel.cs

@@ -44,6 +44,7 @@ namespace Avalonia.Diagnostics.ViewModels
             SelectedTab = 0;
             if (root is TopLevel topLevel)
             {
+                _pointerOverRoot = topLevel;
                 _pointerOverSubscription = topLevel.GetObservable(TopLevel.PointerOverElementProperty)
                     .Subscribe(x => PointerOverElement = x);