Detect if anchor position of segment got out of sync

[philippotto]

If a segment is clicked and the anchor position doesn't contain the id:

• [ ] explain what's happened (segment got out of sync) and how to fix this manually (brush again with the id)
• [ ] offer to remove the segment

Later (as it's more complicated):

• [ ] offer to search for the segment in the surrounding data (expensive and not always feasible)

[normanrz]

At some point, we will want to implement segment statistics. For that, we need to maintain some bboxes of segments in order to constrain volume calculation, surface calculation, and finding center points (or a point). That will make it feasible to search for new positions if the original ones have been erased.

[philippotto]

That's true, but the first two points of this issue are quite an easy fix (I added good-first-issue). So, I wouldn't defer this issue until we got segment statistics.