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".

REVIEW: 118606
Add tests for basic message forwarding (as attachment)

Extend test cases for Composer class to handle subject mangling to
include manglingfor forwarding. Also, add tests to ensure proper
setting of $Forwarded flag for the message that has been forwarded.