Implement visual-order cursor motion.
src/xdisp.c (Fmove_point_visually): New function.
lisp/bindings.el (visual-order-cursor-movement): New defcustom.
(right-char, left-char): Provide visual-order cursor motion by
calling move-point-visually. Update the doc strings.
doc/emacs/basic.texi (Moving Point): Document visual-order-cursor-movement
and its effect on right-char and left-char.
doc/lispref/display.texi (Bidirectional Display): Document move-point-visually.
etc/NEWS: Document the new feature.