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