ETMCalendar: add signal emitted when the user is checking/unchecking a calendar
The goal is to save that new calendar selection right away in korganizer, rather than saving it on clean exit. This way, it's not forgotten in case of a korganizer crash, or session crash or forced shutdown.
The korganizer side of this is simply connect(calendar().data(), &Akonadi::ETMCalendar::calendarSelectionEdited, this, &ActionManager::writeSettings); in actionmanager.cpp