Support for forwarding messages as attachments

Allow forwarding of a message as an attachment by means of menu actions.
Forwarding takes care of mangling the subject line and marking the
original message as "$Forwarded".

Thanks to paalsteek for his initial work on this.

REVIEW: 118606
Add tests for Composer::Util::mangleSubject(..) for Forwarding
Add test to ensure setting of $Forwarded flag after a message forward