Split invitation handler into two.

InvitationHandler is huge and I only want to export a small part
of the API.

InvitationHandler and InvitationHandlerHelper.

InvitationHandler will be called by KMail to process invitations
and will be public API with only one method.

InvitationHandlerHelper is private and will be used by
IncidenceChanger to send invitations whenever we modify, create
or delete incidence. It has lots of methods and we don't know how
to expose them with a clean API yet, so it stays private.