implemented new message notification popup window.
added option: 'Show notification window when new messages arrive'
increased version.
updated po files.
updated NEWS.
modified notification window.
added plug-in API for notification window.
adjusted paddings of notification window.
adjusted paddings.
adjusted paddings of notification again.
ChangeLog: fixed date.
added an option: 'Do not classify message as junk if sender is in the address book'
updated po files.
updated NEWS.
updated Japanese manual.
notification_window_set_message(): check if the window exists.
modified Date header parser.
updated src/libsylpheed-plugin-0.def.
Supported more non-standard date.
optimized date string scan