With Claws Mail's latest double release of 3.18.0 and 4.0.0, the package will refer to the more "modern" GTK+ 3 release, major version four. The GTK+ 2 release, major version 3, is now available in the `claws-mail-gtk2` package. In other words, this commit bumps the GTK+ 2 version from 3.17.8 to 3.18.0, the previously unstable GTK+ 3 version 3.99.0 to 4.0.0 and changes the default to GTK+ 3.
Moved to: ./contributing-to-this-manual.xml. Link: https://nixos.org/manual/nixos/unstable/#chap-contributing