Discovery of replacing display properties now uses the same code

Discovery of replacing display properties now uses the same code
as the display engine.  Tested OK with display properties whose
value is a list.

 src/dispextern.h (struct bidi_it): New member frame_window_p.
 (bidi_init_it, compute_display_string_pos): Update prototypes.
 src/bidi.c (bidi_fetch_char): Accept additional argument
 FRAME_WINDOW_P and pass it to compute_display_string_pos.  All
 callers changed.
 (bidi_init_it): Accept additional argument FRAME_WINDOW_P and use
 it to initialize the frame_window_p member of struct bidi_it.
 src/xdisp.c (handle_display_spec): New function, refactored from the
 last portion of handle_display_prop.
 (compute_display_string_pos): Accept additional argument
 FRAME_WINDOW_P.  Call handle_display_spec to determine whether the
 value of a `display' property is a "replacing spec".
 (handle_single_display_spec): Accept 2 additional arguments BUFPOS
 and FRAME_WINDOW_P.  If IT is NULL, don't set up the iterator from
 the display property, but just return a value indicating whether
 the display property will replace the characters it covers.
 (Fcurrent_bidi_paragraph_direction): Initialize the nchars and
 frame_window_p members of struct bidi_it.