Possible bug: initial SelectedItem is never respected

[GeertvanHorrik]

When assigning SelectedItem via xaml, the item containers are not generated yet, so GetItemContainerForObject returns null. After that, binding to the SelectedItem is impossible because it's not a dependency property.

A possible fix would be:

1. Convert SelectedItem to a dependency property
2. Check if ItemsSource.Count == items.Count, if not, dispatcher and try again later