Fix (most) UniCaseTest failures (fix #27)

Upper- or lowercasing Unicode strings is not possible by mapping one
character to one character in each case, in contradiction of what the
GLib functions g_unichar_tolower() and g_unichar_toupper() would seem
to suggest. The casing operations now operate on the whole string.

Unfortunately, this introduces a new bug, #28, since there is no
title-casing function in GLib that allows for the title-case of a
character to be longer than one character.