title: Qt: Tab Icon when QDockWidget becomes docked
body: <p>Qt's QMainWindow has a ability to dock windows derived from QDockWidget. It also would put one on top on the other if few of them are stacked, producing a tab bar. Whenever a QDockWidget's state changes a signal topLevelChanged() is emmitted. At this point I would like to get access to underlying QTabWidget to set an icon for a a newly added tab. How can I do it? My patience is over trying to dig the answer out from Qt's documentation and source code. Thank you in advance.</p>
  
  <p><a href="https://i.stack.imgur.com/ldpGC.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/ldpGC.png" alt="enter image description here"></a></p>
  
  <p>So icon I want to be on Contents/Index tabs.</p>