Hmmm, well our Glade file targets [GTK 3.20](https://github.com/geany/geany/blob/37d20a823fb1bca67cea57ef73aa9e64009d8138...) and I think there are other things needing later GTKs even though both Autotools and Meson just check for 3.0 and the Mingw CI passes this PR using GTK 3.8 (AFAICT).
So I'm confused what GTK version we _really_ need.