(FRINGE_ID_BITS): New definition for number of

(FRINGE_ID_BITS): New definition for number of
bits allocated to hold a fringe number.  Increase number of bits
from 4 to 8 to allow user defined fringe bitmaps.
(struct glyph_row, struct it): New members left_user_fringe_bitmap,
left_user_fringe_face_id, right_user_fringe_bitmap,
right_user_fringe_face_id.
(enum fringe_bitmap_type, struct fringe_bitmap, fringe_bitmaps):
Move to new file fringe.c.
(MAX_FRINGE_BITMAPS): Define here.
(struct draw_fringe_bitmap_params): New members bits, cursor_p,
and overlay_p.  Change member which to int.
(struct redisplay_interface): New members define_fringe_bitmap
and destroy_fringe_bitmap.
(valid_fringe_bitmap_id_p): Add prototype.
(w32_init_fringe, w32_reset_fringes) [WINDOWS_NT]: Add prototypes.