title: ComboBox with fixed Header
body: <p>I need to show a default text in my <code>ComboBox</code>, this text must not changed also when the user select an item of the <code>Combobox</code>, actually for do this I've created this structure:</p>
  
  <pre><code>&lt;ComboBox ItemsSource="{Binding AvailableNations}" Width="160" Height="55" Margin="0, 0, 0, 15" 
             Text="Select Countries" IsEditable="True"&gt;
       &lt;ComboBox.ItemTemplate&gt;
           &lt;DataTemplate&gt;
               &lt;CheckBox IsChecked="{Binding IsChecked}" Content="{Binding Item.Name}" /&gt;
           &lt;/DataTemplate&gt;
       &lt;/ComboBox.ItemTemplate&gt;
  &lt;/ComboBox&gt;
  </code></pre>
  
  <p>this display as default text <code>Select Countries</code> but if I select an item the default text will disappear and the item selected will be displayed, how can I fix this?</p>