title: Grouping items in a ComboBox
body: <p>I have a ListView that contains two types of objects, single and multiple.
  The single is a ordinary TextBlock while the multiple is a ComboBox with items.</p>
  
  <p>I'm trying to group the items in the ComboBox without success. Is it possible? Or should I go for a different approach?</p>
  
  <p>What I'm trying to achieve:</p>
  
  <pre><code>[ComboBox v]
      [Header  ]
      [    Item]
      [    Item]
      [Header  ]
      [    Item]
  </code></pre>