Handle `(space ...)' display spec as paragraph separator.

Handle `(space ...)' display spec as paragraph separator.

 src/xdisp.c (compute_display_string_pos): Return 2 in DISP_PROP when
 the display spec is of the form `(space ...)'.
 (handle_display_spec): Return the value returned by
 handle_single_display_spec, not just 1 or zero.
 (handle_single_display_spec): If the display spec is of the form
 `(space ...)', and specifies display in the text area, return 2
 rather than 1.
 src/dispextern.h (struct bidi_it): Rename the disp_prop_p member
 into disp_prop; all users changed.
 src/bidi.c (bidi_fetch_char): If compute_display_string_pos returns
 DISP_PROP = 2, substitute the u+2029 PARAGRAPH SEPARATOR character
 for the text covered by the display property.
 lisp/buff-menu.el (Buffer-menu-buffer+size): Remove calls to
 bidi-string-mark-left-to-right; they are unnecessary now.
 doc/lispref/display.texi (Specified Space): Mention that `space' specs
 influence bidi reordering.
 (Bidirectional Display): Explain how to use `(space . PROPS)' for
 separating fields with bidirectional content.