Replace ViewDirection and PlaneOrientation with AnatomicalPlane

API-breaking changes:
Remove the ViewDirection enum from the SliceNavigationController
and use the new AnatomicalPlane enum class for the
different directions / planes.
Remove the PlaneOrientation enum from the PlaneGeometry
and use the new AnatomicalPlane enum class for the
different orientations / planes.

Migration should be easy, since the different directions / orientations
/ planes are the same and in the same order as before.
"PlaneOrientation::None" is now "AnatomicalPlane::Original".