Don't inherit Attendee from Person

Summary:
This fixes the slicing issue for (de)serialization brought up during the
KF5 review. Maybe even more important, this enables us to transform both
classes into simple value types. That would not only be more efficient by
avoiding some extra allocations, it would also enable direct consumption
of those types from QML.

Reviewers: winterz

Reviewed By: winterz

Subscribers: winterz, kde-pim

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D21101