[QOrganizerItemMemoryEngine] Reduce code duplication in saveItem()

This greatly improves the code readability.

Change-Id: I3aa7456b49d5366f1489b774ef546a6e9c923254
Reviewed-by: Michael Krasnyk <michael.krasnyk@gmail.com>
Reviewed-by: Robin Burchell <robin+qt@viroteck.net>