Adapt diff plugin to the new renderer plugin interface

Summary: Unlike previously, this now also handles diffs that have text/plain as mimetype, and by using the standard attachment container you can also easily save the attachment.

Reviewers: knauss

Reviewed By: knauss

Subscribers: #kde_pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D8122