Native line numbers, final testing

[Eli Zaretskii]

I've finished development of this feature and pushed the changes to
the scratch/line-numbers branch.  "Visual" line numbers are
implemented, and all the known bugs should be fixed.  I also added
documentation.

Please test and report any problems you find.  If no grave bugs show
up, I will probably land this on master in a few days.

Thanks to everybody who sent feedback and reported problems and
feature requests.