New file. Move original fringe related declarations

New file.  Move original fringe related declarations
and code from dispextern.h and xdisp.c here.
Rework code to support user defined fringe bitmaps, redefining
standard bitmaps, ability to overlay user defined bitmap with
overlay arrow bitmap, and add faces to bitmaps.
(Voverflow_newline_into_fringe): Declare here.
(enum fringe_bitmap_align): New enum.
(..._bits): All bitmaps are now defined without bitswapping; that
is now done in init_fringe_once (if necessary).
(standard_bitmaps): New array with specifications for the
standard fringe bitmaps.
(fringe_faces): New array.
(valid_fringe_bitmap_id_p): New function.
(draw_fringe_bitmap_1): Rename from draw_fringe_bitmap.
(draw_fringe_bitmap): New function which draws fringe bitmap,
possibly overlaying bitmap with cursor in right fringe or the
overlay arrow in the left fringe.
(update_window_fringes): Do not handle overlay arrow here.
Compare and copy fringe bitmap faces.
(init_fringe_bitmap): New function.
(Fdefine_fringe_bitmap, Fdestroy_fringe_bitmap): New DEFUNs to
define and destroy user defined fringe bitmaps.
(Fset_fringe_bitmap_face): New DEFUN to set face for a fringe bitmap.
(Ffringe_bitmaps_at_pos): New DEFUN to read current fringe bitmaps.
(syms_of_fringe): New function.  Defsubr new DEFUNs.
DEFVAR_LISP Voverflow_newline_into_fringe.
(init_fringe_once, init_fringe): New functions.
(w32_init_fringe, w32_reset_fringes) [WINDOWS_NT]: New functions.