Introduce MemoryCalendar::instance(QString identifier).

We didn't have a way, until now, to get an incidence by
instance identifier.

Instance identifier is the string returned by
Incidence::incidenceIdentifier() which is basically uid+recurringId.

If you had both uid and recurringId, you could just call
calendar->incidence(uid,recurringId), but if you had only the
instanceIdentifier, you couldn't.

instance identifier will be the remote id of akonadi items belonging
to ical resource (and others), currently the remote id is only uid
but that doesn't cut it because know we can have duplicated uids,
with different recurring ids.