Code review comment for lp:~audience-members/audience/gtk-3.12

I have tested this branch a few times, felt stable and working. Just for nitpicking: The diff seems a bit hard to read, given that some changes not necessarily related to GTK-3.12 migration have been added (e.g. CMake file splitting per subdir for data and src, generated Makefile removal), those might be better in separate merges, but necessary. If no one else has objections against those being in this one, I'll give this an approve.