Do a s/babl_destroy/babl_exit since 'destroy' is better paired

Do a s/babl_destroy/babl_exit since 'destroy' is better paired
with a 'create' and other libraries also use the 'init'/'exit'
pair, including GEGL. Keep a define for babl_destroy unless
BABL_DISABLE_DEPRECATED is defined.

* babl/babl-main.h
* babl/babl.c
* tests/babl-html-dump.c
* tests/babl_class_name.c
* tests/babl_fish_path_dhtml.c
* tests/babl_fish_path_fitness.c
* tests/conversions.c
* tests/formats.c
* tests/grayscale_to_rgb.c
* tests/introspect.c
* tests/models.c
* tests/nop.c
* tests/rgb_to_bgr.c
* tests/rgb_to_ycbcr.c
* tests/sanity.c
* tests/srgb_to_lab_u8.c
* tests/types.c

svn path=/trunk/; revision=386