title: Do not allow unselect/deselect in ListBox
body: <p>Is there a way to configure the WPF <code>ListBox</code> such that it's not possible to deselect/unselect an item? So there is always an item selected?</p>
  
  <p>My <code>ListBox</code> binds to an <code>ObservableCollection&lt;MyClass&gt;</code>.</p>