feat(mimetreeparser): Support MemoryHole in MimeTreeParser.

Summary:
The pasring part is done with this patch, but for the user it is not
visible as we need to make sure, that we first create the MimeTree and
afterwards render the header and than the body.

Subscribers: kde-pim

Tags: #kde_pim

Maniphest Tasks: T742

Differential Revision: https://phabricator.kde.org/D23649