title: QDockWidget float/close button hover images
body: <p>Is there a way to change the hover/pressed icons for a QDockWidget(Qt 4.8) in a stylesheet? I'm using this style to change the button image for the default state:</p>
  
  <pre><code>QDockWidget{
      font-weight: bold;
      titlebar-close-icon: url(:/icons/ui-dock-close-off.png);
      titlebar-normal-icon: url(:/icons/ui-dock-float-off.png);
  }
  </code></pre>
  
  <p>I've tried a couple of different ways, but none of them seem to be working.</p>
  
  <pre><code>QDockWidget::float-button:hover{
      image: url(:/icons/icons/ui-dock-float-hover.png);
  }
  
  QAbstractButton#qt_dockwidget_floatbutton{
      image: url(:/icons/icons/ui-dock-float-hover.png);
  }
  </code></pre>