Post 0.6.31 API changes

[Ladislav Michl]

Hi there,

as 6d59b8b1a0f9 ("Move callbacks from gn_data to gn_statemachine->callbacks")
breaks API, we sould probably indicate that in libary version.
Looking to the history, commit 18de817d4958 ("Prepare 0.6.31 release") even
decreased GNOKII_LT_VERSION_AGE, so I'm unsure how is libgnokii meant to
handle API/ABI changes.

I'd like to resurrect GNOME Phone Manager, so something like this is needed:
https://github.com/3x380V/phonemgr/commit/5b35631c7242
Eventually I can just require next gnokii version once (if at all) released.

Side question: libgnokii now depends on glib just to support locales in
gn_sms_send - it uses g_locale_to_utf8 which makes library behavior dependent
on current locale. Should we change to utf8 here?

Thanks,
        ladis