Make KMime::Content available properly in MessagePart
Summary:
Most MessagePart sub-classes were doing this themselves so far, but we
need this generically, e.g. to give render plugins access to header data.
Reviewers: knauss
Reviewed By: knauss
Subscribers: KDE PIM
Tags: KDE PIM
Differential Revision: https://phabricator.kde.org/D8071