title: Unable to bind Accordion SelectedIndex to MVVM integer property
body: <p>My ViewModel needs to know which AccordionItem is selected in the View. So in the ViewModel I have:</p>
  
  <pre><code>public class ServerListControlViewModel : ObservableObject
  {
      private int _accordion_index;
      public int accordion_index
      {
          get { return _accordion_index; }
          set
          {
              _accordion_index = value;
              RaisePropertyChanged("accordion_index");
          }
      }
  }
  </code></pre>
  
  <p>And in the View, I have</p>
  
  <pre><code>&lt;toolkitLayout:Accordion SelectedIndex="{Binding accordion_index}"&gt;
    &lt;toolkitLayout:AccordionItem items go here&gt;
  &lt;/toolkitLayout:Accordion&gt;
  </code></pre>
  
  <p>The problem? The Accordion does the fancy expand/collapse animation, but accordion_index's set method is never getting called. Furthermore, I can set accordion_index to something in the VM's constructor, and the get method will be called when the View is loaded, and my hardcoded value will be returned, but the Accorion ignores it and always defaults to the first item being expanded. Why? I am new to WPF but fairly certain I've bound this correctly.</p>