Extend MessagePart API as needed for the TNEF plugin

Summary:
Mainly NodeHelper and temporary file access. This enabled a bit of cleanup
in the built-in rendering code too, towards getting rid of the friendship
overuse.

Reviewers: knauss

Reviewed By: knauss

Subscribers: #kde_pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D8170