mail-client/balsa: bump to 2.5.6, includes EFAIL security fixes

mail-client/balsa: bump to 2.5.6, includes EFAIL security fixes

*html2text is actually a runtime dependency in the webkit-gtk code paths,
not buildtime for all USE.
* Add glib-compile-resources deps (libxml2).
* New gnutls dep.
* libesmtp usage removed.
* Custom icon cache in /usr/share/balsa, needing gtk-update-icon-cache
  during compile phase.
* src_prepare modifications are all obsolete (upstream bugs fixed).
* Include patches to hopefully keep USE=webkit working with older webkit-gtk.

Closes: https://bugs.gentoo.org/660322
Package-Manager: Portage-2.3.40, Repoman-2.3.9