Add an API to do QT_WIDGETS_RHI_BACKEND

Since there is setGraphicsApi for Qt Quick, maybe similar for Widgets could also be added. Basically it would do what QT_WIDGETS_RHI_BACKEND does.

On Widgets side QT_WIDGETS_RHI & QT_WIDGETS_RHI_BACKEND could be used to avoid windows being recreated when something like QQuickWidget is added to the same window, but this is not documented and not easy to find.