add qRoundReal

https://codereview.qt-project.org/#/c/138524/ brought up this Android issue: https://code.google.com/p/android/issues/detail?id=54418

And in general, sometimes you want to round a qreal without losing precision, and without knowing whether qreal is actually a double or a float. qRound returns an int whereas qRound64 returns a qint64, so you have to know which one you want.

If we have a need to write code like this

template<typename T>
static T Round(T t) {
    return round(t);
}
template<>
float Round<float>(float f) {
    return roundf(f);
}

for one use case, chances are we will need it somewhere else too.