Add a method to QGeoCoordinate which takes an offset and returns another QGeoCoordinate

We currently expose methods which calculate the distance / azimuth (along a great circle) between two QGeoCoordinate instances.
We've been asked to add a method something like
QGeoCoordinate QGeoCoordinate::atBearing(qreal distance, qreal bearing)
and possibly something like
QGeoCoordinate QGeoCoordinate::atDistance(qreal distanceEast, qreal distanceNorth)

The second of these functions could be problematic since the order in which the distances are added will effect the final result. Furthermore, the second function can easily be implemented in terms of the first function, which would force the users to add the distances in an order appropriate to them. I'll update the change request if/when more interest is shown in the second function.

Several users are interested in the first method, which can be added with low technical risk.