Add support for title-casing letters (bug#24603)

Add support for title-casing letters  (bug#24603)

* src/casefiddle.c (struct casing_context, prepare_casing_context): Add
titlecase_char_table member.  It’s set to the ‘titlecase’ Unicode
property table if capitalisation has been requested.
(case_character): Make use of the titlecase_char_table to title-case
initial characters when capitalising.

* test/src/casefiddle-tests.el (casefiddle-tests--characters,
casefiddle-tests-casing): Update test cases which are now passing.