qunicodetools.cpp: libthai's th_brk is not thread-safe [2/2]: new API

The facilities in qunicodetools.cpp are not limited to the GUI thread, so they need to be re-entrant.

Besides QTBUG-105543 there is another threading issue in qunicodetools' thai support: The th_brk() function is apparently not re-entrant, so https://github.com/tlwg/libthai/pull/1 (available from symbol version 0.1.25) added a new, thread-safe, API where the initialization of global state is explicit (and must be protected by the user with a mutex), but following calls to the th_brk replacement are then thread-safe.

We should port to the new API. If we don't find it, emit qWarning() and continue to use the old API.