Make use of cpp.cxxLanguageVersion in the qbs project file(s) once available

[Ablu]

As @bjorn pointed out here: https://github.com/tales/tales-client/pull/30/files#r22762257 there is a nicer way to set the C++ version in newer qbs versions.

This is a reminder to make use of it once the feature is in a release and more widely spreaded.