Native display of line numbers, improved

[Eli Zaretskii <eliz@gnu.org>]

OK, I've updated the scratch/line-numbers branch with several bugfixes
and some new features.

All the bugs reported till now are fixed, with the sole exception of
truncated-lines display.  The latter behaves better now (in
particular, the funny problem reported by Martin, whereby you couldn't
get to the line beginning, is gone), but there are still issues with
hscrolling lines with TABs; I will try to fix those later.  The tricky
problems reported by Alan for Follow mode are also solved (relative
line numbers are now local to each window under Follow mode).

I fixed the issue with TAB stops, but only for the line-number
display, leaving alone other similar issues, like with line-prefix.

I added the features requested by several people, including relative
numbers with current line's number absolute, a separate face for
displaying the current line, and the display-line-numbers-disable
property for company-mode and its ilk.

The only requested feature that remains unimplemented is relative line
numbers counted visually.  The reason is that I'm not sure I
understand the requirements (and also not sure whether people who
asked for that have thought that through).  I will post a separate
message about that.

Comments and bug reports are welcome.  Apart of fixing bugs and
perhaps implementing the visual line counting, the only changes I plan
to commit before landing this on master is proper documentation in the
2 manuals.  Otherwise, the feature is complete from my POV, so here's
your chance to give it a ride before it lands.

Enjoy.