Add a distance function to QPointF

I think the QPointF class should provide a static function to compute the distance between two points. It is very unintuitive to call QLineF(p1,p2).length() just to get distance between two points.

Instead something like 

static qreal QPointF::distance(QPointF const &p1, QPointF const &p2)

should be provided. As a bonus, a "distancesquared" function could also be included.