build: require GTK+ >= 3.1.6
build: require GTK+ >= 3.1.6

We use features from GTK+ master, so we should require it.

ChrisCummins/gnome-themes-standard,rgcjonas/adwaita-semidark,DoctorJellyface/gnome-themes-standard,cosimoc/gnome-themes-standard,rgcjonas/adwaita-semidark,cosimoc/gnome-themes-standard,cosimoc/gnome-themes-standard,ChrisCummins/gnome-themes-standard,rgcjonas/adwaita-semidark,magcius/gnome-themes-standard,ChrisCummins/gnome-themes-standard,magcius/gnome-themes-standard,ChrisCummins/gnome-themes-standard,ChrisCummins/gnome-themes-standard,magcius/gnome-themes-standard,Distrotech/gnome-themes-standard,Distrotech/gnome-themes-standard,DoctorJellyface/gnome-themes-standard,cosimoc/gnome-themes-standard,rgcjonas/adwaita-semidark