WIP: Interface for mail headers in MimeTreeParser

Summary:
To make it possible to display memoryhole correcly to the user,
MimeTreeParser needs to have the ability to update/change headers.
For this we need an interface to change the headers.

Subscribers: kde-pim

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D15760

Maniphest Tasks: T8568