title: Elements starting by center of a stackpanel
body: <p>How can I insert elements into a stackpanel, and they start positioned by center?
  Something like this:</p>
  
  <p>|_ _ x _ _ |</p>
  
  <p>|_ x x _ _ |</p>
  
  <p>|_ x x x _ |</p>
  
  <p>The "x" are the elements, and the "_" are blank space.</p>
  
  <p>Is there something already implemented?</p>