(enum fringe_bitmap_type): Remove. Change all uses

(enum fringe_bitmap_type): Remove.  Change all uses
to use `int'.
(NO_FRINGE_BITMAP, UNDEF_FRINGE_BITMAP, MAX_STANDARD_FRINGE_BITMAPS):
Define explicitly.
(Qtruncation, Qcontinuation, Qempty_line, Qtop_bottom)
(Qhollow_small): New variables.
(syms_of_fringe): Intern and staticpro them.
(question_mark_bits): Rename from unknown_bits.
(left_curly_arrow_bits): Rename from continuation_bits.
(right_curly_arrow_bits): Rename from continued_bits.
(left_triangle_bits): Rename from ov_bits.
(right_triangle_bits): Added.
(filled_rectangle_bits): Rename from filled_box_cursor_bits.
(hollow_rectangle_bits): Rename from hollow_box_cursor_bits.
(filled_square_bits): Added.
(vertical_bar_bits): Rename from bar_cursor_bits.
(horisontal_bar_bits): Rename from hbar_cursor_bits.
(empty_line_bits): Rename from zv_bits.
(standard_bitmaps): Update to use new names.
(draw_fringe_bitmap_1): Make static.
(get_logical_cursor_bitmap, get_logical_fringe_bitmap): New functions
to map from logical cursors and indicators to physical bitmaps.
(draw_fringe_bitmap): Resolve fringe cursor and	overlay-arrow
bitmaps using symbol names instead of bitmap numbers.
(update_window_fringes): Use logical indicator symbol names
instead of bitmap numbers for logical.  Add bitmap cache.
(LEFT_FRINGE, RIGHT_FRINGE): New helper macros.