Log Message:
Update to 2.3.1:

* The compilation failure with GLib 2.4 was fixed.
* The compilation failure in some OSes was fixed.
* A memory leak on executing external editor was fixed.
* The bug that the handling of IMAP4 server responses became
  out of sync when an error occurred while fetching messages
  was fixed.
* "ca-root.crt" is also looked for as a CA certificates file now.
* The spaces around equals between name and value of MIME
  parameters are now allowed.
* The MIME header encoding of quoted multibyte strings was fixed.
* The edit account dialog is now placed at the center of the
  parent window.