dot/emacs: Move the cursor properly.

dot/emacs: Move the cursor properly.

Vertical motion is by visual lines, because this allows more
fine-grained motion -- and not for any deep philosophical reason,
because the arguments from principle are all in favour of motion by
logical lines.

Move the cursor visually horizontally when the cursor keys are pressed,
but in buffer order when the traditional C-b/C-f keys are used.  This
makes most sense to me when I'm trying to edit mixed-direction text.  It
probably won't suit anyone else.