When using multiple identities, and creating a new event with attendees,
the identity selected as organizer should be used in order to use the corresponding Sent folder.

Of course when editing an event created by someone else, this lookup will fail, and we'll
fall back to the global korganizer setting instead, as before. Hmm, maybe we should look up
which identity of ours is an attendee? Not sure about the use case though, probably only
shared calendar folders.

Created an event with a non-default identity, invited someone else,
now the mail is stored in the correct Sent folder for that identity.