Move preference out of type for Email

Summary:
Preference being a type flag is vCard 3.0 style, preference being a
separate parameter is vCard 4.0 style.

This does not change how we actually serialize this, just make the API
consistent with e.g. ResourceLocatorUrl and Impp.

Reviewers: mlaurent

Reviewed By: mlaurent

Subscribers: mlaurent, kde-pim

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D21019