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).