Use new-style GTK+ deprecation defines

Target GTK+ 3.4, as that is the version of GTK+ 3 that is in Debian
stable, which a future GTK+ 3-only version of EasyTAG can comfortably
depend on. Ignore deprecations in versions of GTK+ 3 > 3.4. Keep the
GTK+ 2 behaviour the same, as no new deprecations should be added.

https://wiki.gnome.org/HowDoI/Deprecations