New modules for grapheme cluster breaking.

New modules for grapheme cluster breaking.

New module 'unigbrk/base'.
* modules/unigbrk/base: New file.
* lib/unigbrk.in.h: New file.

New module 'unigbrk/uc-gbrk-prop'.
* lib/gen-uni-tables.c: Generate lib/unigbrk/gbrkprop.h.
* modules/unigbrk/uc-gbrk-prop: New file.
* lib/unigbrk/gbrkprop.h: New file.
* lib/unigbrk/uc-gbrk-prop.c: New file.

New module 'unigbrk/uc-is-grapheme-break'.
* modules/unigbrk/uc-is-grapheme-break: New file.
* modules/unigbrk/uc-is-grapheme-break-tests: New file.
* lib/unigbrk/uc-is-grapheme-break.c: New file.
* tests/unigbrk/test-uc-is-grapheme-break.c: New file.
* tests/unigbrk/test-uc-is-grapheme-break.sh: New file.
* tests/unigbrk/GraphemeBreakTest.txt: New file.

With corrections and tweaks by Bruno Haible <bruno@clisp.org>.