Prepare for Attendee becoming a value type

Summary:
This ports away from relying on Attendee pointers to modify existing
attendees inside incidences, but uses explicit setters instead.

Reviewers: KDE PIM, mlaurent

Reviewed By: KDE PIM, mlaurent

Subscribers: kde-pim

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D21717