fix(mimetreeparser): Make it possible to distinguish different encrypted/signed message parts.

Summary:
This is mostly important for inline encrypted messages, as they don't have
a corresponding mime part.

That should finally make rendertest be happy again.

Test Plan: run rendertest

Reviewers: #kde_pim, gjditchfield

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D23396