Set the correct last modified time when creating an exception for a recurring event.

I'm proposing this MR from a patch provided by Boris Shmarin in SailfishOS : https://github.com/sailfishos/kf5-calendarcore/pull/5

It's about an issue when creating exceptions with Calendar::createException(). If the parent has lastModified set to a date time, then the exception is inheriting it from the clone() call. But only the created time stamp is set to the current date in the createException() routine. It results that the lastModified time stamp will be earlier in time than the created one. This may confuse some sync servers (typically an Exchange calendar as reported above).

I've added some QVERIFY in the corresponding test to highlight the issue.

@winterz may I ask you to review this MR ? Thank you.