Handle incidences in different time zones in MemoryCalendar.

Summary:
a calendar and an incidence may be given in different
time zones. This may confuse the date cache in MemoryCalendar.
Properly move to the calendar time zone when setting incidences
in the date cache.

Test Plan:
I've added a method in testmemorycalendar to demonstrate
the issue, using a UTC calendar and adding an event from a time
zone where the date as already changed. I've added in this test
various possibility to track addition, modification and deletion
of such events.

Reviewers: KDE PIM, winterz, vkrause, dvratil

Reviewed By: KDE PIM, dvratil

Subscribers: KDE PIM

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D25066