Below is a patch to display and print the headers of embedded
message/rfc822 parts (such parts are usually a result forward->attached
messages). To enable re-using of existing code, I moved the displayed
headers into a new structure which is used from both libbalsa/message.[hc]
(for the "main" headers) and libbalsa/body.[hc] (embedded). This causes
many tiny changes in a lot of files, making this patch so big.

I tried to be extra paranoid to check if the headers structure is present
when needed. These checks will usually be useless, as the structure will be
there as long as memory is available (and maybe no very strange race
conditions occur?), but I like to be safe... Of course you could remove them.

Details:

- libmutt/parse.c:
make libmutt extract headers from embedded messages

- libbalsa/libbalsa.h:
typedef LibBalsaMessageHeaders

- libbalsa/message.[hc]:
define & use LibBalsaMessageHeaders, define some helper functions, define
the infrastructure to create and destroy the new struct, handle headers
being moved to the new (sub-)struct

- libbalsa/body.[hc]:
add headers to the body struct and take care to allocate, set and free them

- libbalsa/filter.c, libbalsa/send.c:
handle headers being moved to the new struct

- src/balsa-message.c:
move stuff to display headers to a new function (display_headers_real)
which is called via wrappers to render both the main and the embedded
headers, handle headers being moved to the new struct

- src/print.c:
same for printing

- src/balsa-index.c, src/sendmsg-window.c, src/store-address.c:
handle headers being moved to the new struct