Log Message:
Update to sylpheed-3.0.0.
Many bugfixes. New features include:
* multi-threading was introduced
* add new recipients to address book automatically on sending was added.
* The new filter match type 'is in addressbook' was added.
* Address book is now sortable by each column.
* Address book search feature was added.
* The setup dialog on the first run became user-friendly.
* The new account setup dialog was implemented. Easy Gmail setup.
* SHA1/MD5 fingerprint and validity period of certificate is now displayed
* menu item to request disposition notification was added
* The feature to customize the color label text was added.
* The option to set only mail address of recipients when replying was added.