Add a version of split() to QString that returns QList<QStringRef>

I think a significant performance boost in text parsing can be achieved if a kind of split() is added to QString that returns QList<QStringRef>.

The attached QTest project contains microbenchmarks to compare QString::split() with a similar function using QStringRef. I have observed execution speed increases from 15% to 35% compared to QString::split() on different setups. I think even %15 improvement is compelling assuming that implementing such a new member function is rather simple.