This is an elaborate hack to get multi-calendar support.
All uid's in memory calendar are namespaced by calendar, so the same event
can appear multiple times in various Akonadi::Collection's.
This was so far not handled at all.
A cleaner solution would either move all this functionality into KCalCore::Calendar,
since this patch breaks most operations that work with uid's directly,
or make the the MultiCalendar a wrapper for several KCalCore::Calendars.