title: Insert QWidget to an existing QBoxLayout at a particular position?
body: <p>The questions says it all. Say I have a layout</p>
  
  <pre><code>mylayout
  |-- widgetA
  `-- widgetB
  </code></pre>
  
  <p>How do I insert a new widget to this tree?</p>
  
  <pre><code>mylayout
  |-- widgetA
  |-- widgetC
  `-- widgetB
  </code></pre>