Adds code to sfx2 and package to try gpg4libre for extracting
session keys, and use them in turn to decrypt odf storage.

Change-Id: I1f626143e6c8443b4ad0c4fc5bdbd5ab8d56a451
Reviewed-on: https://gerrit.libreoffice.org/47780Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Thorsten Behrens <Thorsten.Behrens@CIB.de>