Add support for simulating mouse wheel events in QTest

QTest has functionality to simulate mouse clicks and movements but there is no way to simulate mouse wheel events. Looking at documentation (http://doc.qt.io/qt-5/qtest.html#details) there is not a single mention of "wheel" anywhere.

Suggestion is to add this basic functionality to QTest.

This has been requested recently (https://stackoverflow.com/questions/50996997/how-to-simulate-mouse-wheel-events-using-qtestlib-qt5) and as far back as 10 years ago, in  2008 (https://www.qtcentre.org/threads/13436-Wheel-and-QTest).

Today this request was made formal.

NOTE: If there is a misunderstanding that this functionality is in fact available, then the documentation should be updated on how to do it!