Implement and use libbalsa_wrap_quoted_string()

See #74 (closed) When a text/plain part is not Format=Flowed and is wrapped to a user-specified width, each wrapped line should be quoted in the same way as the original line, both when displayed and when quoted in a reply.

modified: libbalsa/mime.c modified: libbalsa/mime.h modified: src/balsa-mime-widget-text.c modified: src/sendmsg-window.c

Closes #74 (closed)