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