check the return value of junk filter command and show error dialog on failure.
Automatically select appropriate Junk folder on filtering.
added a new filter match type: is (not) in addressbook.
display error dialog if the execution of the junk filter command failed.
fixed IMAP4 filtering when IP unreachable.
merged from libsylph branch.
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).