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