Display "multipart/signed" messages like plain-text messages

This lets me show and reply to signed messages just like normal, but
doesn't give me any indication that there's actually a message on the
inside.
Strip the last line (if blank) from MIME parts

So it appears that I'm supposed to strip this, based on my attempts to
verify a GPG signature from myself.  When comparing the results
against claws-mail I had an extra newline.
Remove an extra blank line
Replace "fold" with my own code

I guess this had to be done at some point, but it's really necessary
now to make GPG output pretty print correctly.
Verify (and display) GPG signatures

This is actually fairly simple, after making sure to strip the extra
blank line and get all the newlines correct.  The big change here is
that I now output some ASCII control codes in order to put the signed
message (or improperly signed message) inside a box.