title: Set TextSearch.Text for WPF-ComboBoxItem via DataTemplate
body: <p>I use a datatemplate to visualize some items in a ComboBox,
  the ItemsSource is bound to an ObservableCollection. 
  To keep it simple, let's say I put persons into the ObservableCollection:</p>
  
  <pre><code>public class Person {
      public string FirstName { get; set; }
      public string LastName { get; set; }
  }
  </code></pre>
  
  <p>My DataTemplate looks like this:</p>
  
  <pre><code>&lt;DataTemplate TargetType="{x:Type Person}"&gt;
    &lt;StackPanel Orientation="Horizontal"&gt;
      &lt;TextSearch.Text&gt;
        &lt;MultiBinding StringFormat="{} {0} {1}"&gt;
          &lt;Binding Path="FirstName"/&gt;
          &lt;Binding Path="LastName"/&gt;
        &lt;/MultiBinding&gt;
      &lt;/TextSearch.Text&gt;
      &lt;TextBlock Text="{Binding FirstName}" Margin="2,0" /&gt;
      &lt;TextBlock Text="{Binding LastName}"/&gt;
    &lt;/StackPanel&gt;
  &lt;/DataTemplate&gt;
  </code></pre>
  
  <p>Now I want to enable autocompletion for full names in the ComboBox without introducing a third property on my person class. Hence I do not want to use TextSearch.TextPath Property on the ComboBox, but instead I would like to bind the TextSearch.Text-Property of each ComboBoxItem in the DataTemplate.
  Unfortunately, when I do this (which works with a MultiBinding and StringFormat, tested with Snoop), the bound value is registered for my StackPanel only, but using Snoop (great tool) I found that this stackpanel serves just as  content for some further ComboBoxItemTemplate, which puts another border etc. and finally a ComboBoxItem-tag around my outer StackPanel. Therefore, the TextSearch.Text-setting is not effective, because it must be set in the created ComboBoxItem, and not somewhere within.</p>
  
  <p><strong>Question now:</strong> How can I propagate this TextSearch.Text-Property from my DataTemplate to the surrounding ComboBoxItem using XAML-Styles and -Control-Templates only?
  The solution may modify the default-ControlTemplates of ComboBox and ComboBoxItem and my custom (Item-)DataTemplate, but not use any Code-Behind, or at least not too much. Maybe an attached behaviour would be ok, too. But I'm almost sure there must be a way to make it work without, TemplateBinding or RelativeSource-stuff...
  And of course the solution must make my keyboard-selection and textcompletion work, s.th. when the list contains Hans Josef and Hans Peter, then entering 'Hans ' should autosuggest Hans Josef, whereas entering 'Hans P' quickly enough should autosuggest Hans Peter.</p>
  
  <p>Any solutions?</p>