Add claws-mail and libetpan.

Adds Nix expressions for claws-mail (http://www.claws-mail.org) and a
dependency that is not already in the tree, libetpan
(http://www.etpan.org/libetpan.html).

The Claws expression has flags for toggling the build of various plugins.
Merge pull request #4381 from khumba/claws-mail

New packages: claws-mail-3.10.1, libetpan-1.5.