Summary:
With this it's now possible to implement the diff highlighter using the
new API. However, it's far from convenient still as we can't easily
re-use Grantlee template fragments for the attachment containers yet.

Reviewers: knauss

Reviewed By: knauss

Subscribers: #kde_pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D8114