Make benchmarks compile without QtWidgets

Make benchmarks compile without QtWidgets

Ensure benchmarks which need QtWidgets are gracefully disabled when
that module is unavailable.

Fixed one unnecessary usage of "QT+=widgets".

Change-Id: I8031b5dca585749f0f4d22e0637adc3f57f4e418
Reviewed-by: Kalle Lehtonen <kalle.ju.lehtonen@nokia.com>
Reviewed-by: Sergio Ahumada <sergio.ahumada@nokia.com>