title: h:selectmanyListbox make "choose" option unselectable when at least one option is selected
body: <p>Is it possible to have in <code>&lt;h:selectManyListbox&gt;</code> a default option like "--choose--" which can be selected when no option is selected. When the some value is chosen, then it must be unselectable.</p>
  
  <pre><code>&lt;h:selectManyListbox value="#{bean.value}"
          class="form-control"&gt;
          &lt;f:selectItems value="#{bean.dropdownValues}" var="value" itemLabel="#{value}" itemValue="#{value}"/&gt;            
  &lt;/h:selectManyListbox&gt;
  </code></pre>