title: changing stylesheet dynamically
body: <p>I'm trying to change the style of my QLabel using a dynamic property since we can target this property in QSS like this:</p>
  
  <pre><code>QLabel[foo = "warning"]{ color: red; }
  QLabel[foo = "success"]{ color: green; }
  </code></pre>
  
  <p>the stye of my label does update when I use the <code>QApplication::setStyleSheet()</code> but does not seems to work when I change the value of the property.</p>
  
  <pre><code>label = new QLabel( this );
  label-&gt;setText( "some text" );
  label-&gt;setProperty( "foo", "warning");
  
  // after some event
  label-&gt;setProperty( "foo", "success" );
  // the label's color should be green now
  </code></pre>
  
  <p>did I missed something, or the style change can just not work this way ?.</p>