CoreProfile WidgetRendering on OSX

This bug: https://bugreports.qt.io/browse/QTBUG-32639

also affects OpenFlipper free, when a coreProfile context is used. The dirty workaround, to set the WindowOpacity helps, but i highly discourage the use of any Widgets, that use the PaintEngine mentioned in the bug report, when a CoreProfile is used with Qt. In general doing that almost always is a huge pain in the code.

Any widget, that is rendered on top of the gl area, (e.g. the move picking menu, the general picking menu etc.) is affected by this bug.