WIP: Migrate from intltool to gettext

This is a follow-up on the patch from #279 (closed).

I've mode some extra necessary changes, but I'm still struggling with getting gettext recognizing the local .its and .loc files. I don't know whether it's due to some error in autotools or in the GETTEXTDATADIRS env variable, but I can't seem to make it work.

Anyone with more experience willing to help?