added filter conditions of status such as 'unread', 'mark', 'color-label', and 'mime'.
implemented query search and virtual folder (merged from newsearch branch).
added utility functions for optimization of filter.
changed the license of LibSylph from GPL to LGPL.
moved more modules to libsylph.
moved procmsg.c::procmsg_get_filter_keyword() to filter.c::filter_get_keyword_from_msg().
moved two functions from prefs_filter.c to filter.c.
moved prefs_filter_read_config() to filter.c.
integrated junk filter setting into common prefs.
Initial import of Sylpheed (GTK2 version).