Moving squaredDistanceToSegment to the coordinate package
Do not split segments while in the loop

In combination with the additional squaredDistanceToSegment
check we can ensure that a future vertex is only created on one
segment per feature, and only on the segment that it appears on.

This change also renames the misleading dragVertices_ member to
dragSegments_.
Filter by pixel tolerance before showing the future vertex
Handle the case where the feature is not in the tree