Message structure major(?) change

[Pawel Salek <pawsa theochem kth se>]

Hi,

I think the Message's body_list should be replaced by GNode to reflect
more accurately both the libmutt data structures as well as the real
message. This would simplify considerably handling of multipart
messages, including, but not limiting to, messages with attachments. I
will implement this change except I am treading on someone's toes
again.

Pawel
PS.
concerning the wrong numbers in the status bar, the shown value as
absolutely static, changed only when switching between mailboxes, I think.
Furthermore, it's overwritten with the "Checking Finished"message in the
multhi-thread build so I guess we need some order/rules, how the messages
in the status bar are to be changed.

Message renumbering would be nice, I would add it to the TODO list.