change return type of RotationItem::rotation

In core, we have class IRotation and children to describe various rotation operations, and class RotMatrix (wrapping a class from libheinz) to describe the same in uniform way through a matrix, which in turn is parameterized by quaternions. Maybe we should merge these two descriptions in the future. For the moment, the least we can do, is to distinguish them clearly and cleanly.

In class RotationItem, we have a function rotation, which does not return an IRotation*, but a RotMatrix. It should be replaced by a function rotation that returns an IRotation*. With that, in ItemWithParticles::createRotation we can use IRotation::clone instead of passing to matrix description and back.

If the matrix is needed, it can be created indirectly by item->rotation()->rotMatrix(). Therefore, there is no place in RotationItem for a function that directly returns a RotMatrix.