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.
26f52bf6b5