Fix crash when opening e-mail with calendar invitation

Commit d82e7001 introduced use of new loading state API, which has unfortunately caused a regression. The reason for it is a constructor circular dependency. The FetchJobCalendarPrivate class constructor is trying to call a method of a base class, which hasn't been constructed at this point, as the CalendarBase constructor has not executed yet waiting for the FetchJobCalendarPrivate constructor to finish.

Untangle this mess by moving the method call to the FetchJobCalendar constructor, at which point the base class is already initialized.