Initial version of string reordering. Tested only on mode line and

Initial version of string reordering.  Tested only on mode line and
TTY menu bar.  Display is buggy.

 src/xdisp.c (next_element_from_buffer): Improve commentary for when
 the iterator is before prev_stop.
 (init_iterator): Initialize bidi_p from the default value of
 bidi-display-reordering, not from buffer-local value.  Use the
 buffer-local value only if initializing for buffer iteration.
 (handle_invisible_prop): Support invisible properties on strings
 that are being bidi-reordered.
 (reseat_to_string): Enable bidi-related code.
 (set_iterator_to_next): Support bidi reordering of C strings and
 Lisp strings.
 (next_element_from_string): Support bidi reordering of Lisp strings.
 (handle_stop_backwards): Support Lisp strings as well.
 (display_mode_line, display_mode_element): Temporarily force L2R
 paragraph direction.
 (display_string): Support display of R2L glyph rows.  Use
 IT_STRING_CHARPOS when displaying from a Lisp string.