Mouse's thumb buttons don't work in wizards

1. Use a five button mouse.
2. Build trivialwizard example ( qtbase\examples\widgets\dialogs\trivialwizard\trivialwizard.pro ).
3. Start the example and click "Next".
4. Press the rear thumb button, the mouse shortcut for the "back" button.
   Nothing happens.
5. Press the front thumb button, the mouse shortcut for the "next" button.
   Nothing happens.

The shortcuts which work in native dialogs should work in Qt dialogs, too. This is the last remainder of QTBUG-35203.