Changes:
* The outgoing encoding can be changed on composition temporarily
now.
* The folder selection dialog was reimplemented using GtkTreeView.
* An option that specifies whether to render HTML as text or not
was added.
* The state of the expander of the file save dialog is now
remembered.
* Every message-ids in References: header is now used for message
threading.
* The internal code conversion API has been changed.
* A possible crash on re-editing draft, queue and sent messages was
fixed.
* The error checking of Japanese code conversion also became strict.
* An empty Reply-To: header is now ignored.