Sign/encrypt iTIP emails
This change implements support for signing and/or encrypting iTIP emails. The code uses the MessageComposer::Composer to compose the signed/encrypted message, so the same code as KMail to avoid duplicating all the complex message-building logic. There's still a lot of code duplication between this and the MessageComposer::ComposerViewBase class, but that's not easily avoidable without larger refactoring of the ComposerViewBase and further separating UI & logic.
It honors the organizer's crypto preferences (which is stored in the Identity used for sending the iTIP email) as well as crypto preferences of all attendees (which are loaded from KAddressbook).
The overall UX of this feature is kind of incomplete - there's no way for the user to enable singing or encryption manually like in the KMail composer and also there's no indication in the Incidence Editor UI whether the iTIP email will be signed or encrypted. This can be addressed separately in the incidenceeditor later, thought and I think it doesn't prevent this feature from being merged. Also the message viewer in KMail is not showing the pretty iTIP preview, just shows the cal.ics file as an attachment.