[KJots] Port to NoteMessageWrapper

Summary:
Drop code that interacts directly with KMime::Message, and use handy NoteUtils::NoteMessageWrapper instead.
It also allows setting of lastModifiedDate (which will become useful useful as sort role).
Also, drop DocumentCursorPositionRole role for model, which is not used anywhere.

Note that there is large code overlap between NoteCreatorAndSelector and LocalResourceCreator.
Will do something with it in the following patches.

Test Plan: Code compiles, new books / pages are still created

Reviewers: dvratil

Reviewed By: dvratil

Subscribers: kde-pim

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D29217