title: How to add a "Select one..." option in ZK combobox
body: <p>I'm using the ZK framework and I want to populate a combobox with the values of a given list , but I also want to include a "Select One..." option, as the combo is not required to have a value on submit.</p>
  
  <p>My view looks like this:</p>
  
  <pre><code>&lt;row&gt;
      &lt;hbox&gt;
          Items:
      &lt;/hbox&gt;
      &lt;combobox width="100%" readonly="true" model="@load(vm.itemList)" selectedItem="@bind(vm.object.item)" &gt;
          &lt;template name="model"&gt;
              &lt;comboitem label="@load(each.description)" /&gt;
          &lt;/template&gt;
      &lt;/combobox&gt; 
  &lt;/row&gt;
  </code></pre>
  
  <p>In the ViewModel I retrieve the list with a record set from the database and I don't want to add a record just for this and, if possible, I also want to avoid adding an empty Item on the list.</p>
  
  <p>I think that should be possible to do something like this:</p>
  
  <pre><code>&lt;combobox width="100%" readonly="true" model="@load(vm.itemList)" selectedItem="@bind(vm.object.item)" &gt;
      &lt;template name="model"&gt;
          &lt;comboitem label="@load(each.description)" /&gt;
      &lt;/template&gt;
      &lt;comboitem label="Select One..."/&gt;
  &lt;/combobox&gt; 
  </code></pre>
  
  <p>But neither that or this works:</p>
  
  <pre><code>&lt;combobox width="100%" readonly="true" model="@load(vm.itemList)" selectedItem="@bind(vm.object.item)" &gt;
      &lt;template name="model"&gt;
          &lt;comboitem label="@load(each.description)" /&gt;
          &lt;comboitem label="Select One..."/&gt;
      &lt;/template&gt;
  &lt;/combobox&gt; 
  </code></pre>