Windows: QApplication::widgetAt fails when screen sharing app is running

When using an application like Join.Me (https://join.me/) or Microsoft Lync with screen sharing features, the API QApplication::widgetAt fails to return the widgets for your application.

Join.Me's screen sharing app creates an invisible window with WS_EX_TRANSPARENT flag set. The window covers the entire screen space for the monitor being shared. QApplication::widgetAt first attempts to find the top level window by using ChildWindowFromPointEx but doesn't specify to ignore transparent windows. This is different from 4.8.x where it used WindowFromPoint.

See http://blogs.msdn.com/b/oldnewthing/archive/2010/12/30/10110077.aspx