Implement visual-order cursor motion.

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.