auto-hscroll only the current line in a window

[Eli Zaretskii]

Someone on reddit got excited about such a feature in Nano, so I came
up with its emulation, see the latest trunk.  Give it a try and see if
you like it.  It looks cool, but I cannot really think why would I
need it in Real Life™.

Caveat: the feature might have some rough edges, and I expect a few of
them to be insoluble, since some parts of the display engine simply
cannot work 100% correctly with this feature, without a thorough
redesign.  But it seemed to work surprisingly well in my testing, so
maybe the problems only rarely rear their head.  Report any problems
you see, of course.

Enjoy.