Give legacy BPF::format() handling its own MessagePart sub-class
Summary:
This allows us to remove all infrastructure needed for legacy handling
from the MessagePart base classes. Also, with Interface::MessagePart
vs MessagePart now no longer having any special meaning, we can merge
those two in a next step.
Reviewers: knauss
Reviewed By: knauss
Subscribers: KDE PIM
Tags: KDE PIM
Differential Revision: https://phabricator.kde.org/D8066