made export/import progress dialog cancellable.
reflect account preference for trash folder.
cancel online mode switch if active sessions exist.
disconnect all current sessions of remote folders when going offline.
added 'Junk' special folder and automatically create it when not exist.
add a new flag: RemoteFolder::remove_cache_on_destroy.
update summary caches on receiving.
modified the method of getting IMAP4 folder list.
added 'To' column to the summary view.
the quick search condition now persists for the whole session.
implemented the automatic filtering of IMAP4 INBOX folder.
added 2 Folder API.
added the option 'Remember last selected message'.
libsylph/folder.h: fixed comments.
implemented query search and virtual folder (merged from newsearch branch).
enabled the property of the top folder.
changed the license of LibSylph from GPL to LGPL.
added 'View/Sort/by thread date'.
moved more modules to libsylph.
implemented folder move.
optimized the writing of mark file.
optimized writing summary cache.
reimplemented folder view with GtkTreeView.
Initial import of Sylpheed (GTK2 version).