Preview v2021: snapping end and center points

This is a function, which has been requested by 2D customers, but it might be useful for 3D users as well: Since long 3DViewStation has a behaviour, that it starts snapping to characteristic points, when you start moving your mouse cursor close to it. I.e. when you do a point to point measurement, we might snap to an edge, which is close to your cursor. We now allow to limit the snapping to the centerpoint of the closest line or to an endpoint.