ToDo on next major release:

- offset, length or pos arguments of gcstring_setpos(), gcstring_shrink(),
  gcstring_substr(), gcstring_replace(), gcstring_lbclass() and
  gcstring_lbclass_ext() would be ssize_t, not int.

- Following functions would be deprecated:
  - linebreak_lbrule
  - linebreak_set_user and user_func/user_data members
  - linebreak_lbclass
  - linebreak_eawidth

- Separated libraries for character database, grapheme cluster segmentation
  and line breaking.

