Initial support for visually-relative line numbers

Initial support for visually-relative line numbers

Works very slowly.

* src/xdisp.c (display_count_lines_visually): New function.
(maybe_produce_line_number): Support 'visual' mode of line-number
display.
* src/xdisp.c (maybe_produce_line_number): Update IT's metrics
also when glyph_row is NULL.  This is important for move_it_*
functions.
(syms_of_xdisp) <display-line-number-width>: Now buffer-local.
(try_window_id, redisplay_window, try_cursor_movement): For
'visual' line-number display, disable the same redisplay
optimizations as for 'relative'.

* lisp/cus-start.el (standard): Add new value for the
customization form of display-line-numbers.