Delete some mails with gnubiff (GUI)

Forwarded feature request to upstream:
https://bugs.launchpad.net/ubuntu/+bug/204645

Sometimes I want to delete some mails immediately after I see them in
gnubiff (spam or stupid mails) and don't want to start my mail-client for
that (it's to annoying to start a big program for that minor issue). So I
request a little "delete this mail" button in the preview-box.

Maybe a little red cross on the right side of the overview or on the top in
the message-preview (hold the mouse key pressed... slide over that
delete-symbol and release the mousebutton).