title: TabControl.ItemTemplate: set TabItem.Header.Text to a MultiBinding with StringFormat
body: <p>How do I set the TabItem.Header to bindings taken from few fields, each binding shown in a different size, all in the place of the original header text; without overriding the default style and behavior of the header - I only need the text.</p>
  
  <p>I tried to set its template but then it creates a rectangle that contains the inner controls, and this rectangle is not responsive for user clicks, and also have the control-style, I want this controls to be invisible, only its text should be visible.</p>
  
  <p>I've tried the following:</p>
  
  <pre><code>&lt;TabControl ItemsSource="{Binding}"&gt;
      &lt;TabControl.ItemTemplate&gt;
           &lt;DataTemplate&gt;
               &lt;TabItem&gt;
                   &lt;TabItem.Header&gt;
                       &lt;MultiBinding StringFormat="{}{0}-{1}"&gt;
                           &lt;Binding Path="Title"/&gt;
                           &lt;Binding Path="Category.Title"/&gt;
                       &lt;/MultiBinding&gt;
                   &lt;/TabItem.Header&gt;
                   &lt;TabItem.Content&gt;
                       &lt;TextBlock&gt;
                           Here is what is gonna be in the TabItem - not header
                       &lt;/TextBlock&gt;
                   &lt;/TabItem.Content&gt;
               &lt;/TabItem&gt;
           &lt;/DataTemplate&gt;
      &lt;/TabControl.ItemTemplate&gt;
  &lt;/TabControl&gt;
  </code></pre>
  
  <p>But it doesn't show anything.</p>
  
  <p>I've also tried to set the HeaderTemplate to a DataTemplate but what happens is, the DataTemplate overrides the TabItem style and when I click the text it doesn't go to the clicked tab, besides, the unselected tabs look very funny, I see the rectangle of the text floating, while I want it to be transparent.</p>
  
  <p><strong>So, to summarize up my question</strong>, I want to set TabItem.Header.Text to a MultiBinding with StringFormat.</p>