title: Qt: query QWidgets on the actual visibility on the screen -- not "isVisible()", more like "isRendered()"
body: <p>I'm thinking about generating lists of certain Qt-Objects that are rendered/displayed inside a certain QWidget (or even the complete QWindow) at a given time.</p>
  
  <p>Using traversing the list returned by "QObject::children()" and the Qt-property "visible" does not really help: QWidgets might have set "visible" to "true", but are not visually exposed in the current screen-layout. QActions inside a closed QMenu for example.</p>
  
  <p>Are there other ideas to access Information like this during run time? In the moment I fear that touching the Qt-source-code and hooking into some rendering-functionality might be an option...</p>
  
  <p>What seemed to work "more or less" is using QApplication::widgetAt(QPoint) iterating over all points inside the current window. But this is unusable slow for real-time application...</p>
  
  <p>Motivation: I would like to "measure" the complexity of an (Qt-)Application over time based on the type, area and number of displayed elements. I know this is a rather complex task... but pen&amp;paper is not really a viable option ;-)</p>