title: ListBox (multiselect) selected indexes
body: <p>I have a <code>ListBox</code> which supports <code>Multi-Select</code> and I need a way to retrieve all the selected indexes as a <code>List&lt;int&gt;</code>.</p>
  
  <p>for example:</p>
  
  <pre><code>Listbox
  {
      item 1 - not selected
      item 2 - selected
      item 3 - selected
      item 4 - not selected
  }
  </code></pre>
  
  <p>So the selected indexes <code>List&lt;int&gt;</code> will look like:</p>
  
  <pre><code>List&lt;int&gt;() { 1, 2 };
  </code></pre>