Calendar:Lightning:Invitations and inbox
- iMIP/iTIP inboxes (such as in CalDAV) could be merged with the mail inbox
- iMIP/iTIP attachments from email messages "promoted" as well?
- How to show the different invitation states? (NEEDS-ACTION, DECLINED, ACCEPTED)
- Do we care about TENTATIVE/CONFIRMED here?
- Should use icon/color to show an invitation that conflicts with an existing event