Paint over childs

[SABROG]

I have QMainWindow and reimplement paintEvent. When showing modal dialogs i want shade whole main window, how this do on more sites like http://www.crossplatform.ru/ when clicking on screenshots, but i can't find info how to paint over childs (QWidgets).
I would not want to override paintEvent of all childs...

I hear about solution with transparent QWidget (as Layout) and raise()/lower(), but this sounds like as overhead.

P.S.: First picture - original, Second - shaded (what i want), Third - what i have. (i don't have Photoshop and use invert colors on 2nd picture instead shade).