Gui: add icon for mail forward

icons/mail-forward is from oxygen's actions/32x32/mail-forward.png
licensed under LGPLv3

The name mail-forward is retained to allow picking up of the
relevant icon from a user's theme/icon-set other than oxygen.
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.

BUG: 321378
REVIEW: 118606
Tests: Add test to ensure setting of $Forwarded flag after a message forward
Tests: add test rows for reply subject mangling to handle "Fwd"s
Tests: add test for subject mangling when forwarding message