This operation is the exact opposite to QQuaternion::fromAxisAndAngle()
(so that it is a way to extract the axis and angle values suitable
to create the same quaternion via QQuaternion::fromAxisAndAngle()).
Change-Id: I41fda58f5fb2b867cccd6b2faf58ab671fa070da
Reviewed-by: Laszlo Agocs <laszlo.agocs@theqtcompany.com>