Segment related functionality can be removed from Synfig

Hello,

This is a note for @devs.
Segment related functionality (Segment Vertex, Segment Tangent, supporting code in valuenode_bline.cpp, loadcanvas.cpp, etc) can be safely removed from Synfig.
Segment is a two point cubic spline.
There are three reasons for removal:

  1. It is impossible to use it now since user cannot create a segment, so it’s completely useless.
  2. It appears that segments were a transitioning converters that later became “BLine Tangent” and “BLine Vertex”. It served the same purpose but was much more limiting. So one reason to keep it would be backwards compatibility, but there’s no work that utilizes this converter. I obviously didn’t scan all of sif files in the world, but I am pretty certain that it never got into stable branch so this converter was never used in user works.
  3. Speaking of users, since “Segment Vertex/Tangent” converters don’t do anything it might confuse people who will try experimenting with it because, you know, documentation doesn’t really tell you that you’re wasting your time.

Overall, I think it’s a good idea to remove it.