Add email type and preference API

Summary:
This is used in several places in KDE PIM via string-based access.
So let's upstream this too and give it a proper API, along the lines
of how this is done for Address or PhoneNumber.

Reviewers: dvratil

Reviewed By: dvratil

Subscribers: dvratil, kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D20497