Deletion should update message window

[Andrew Conkling <andrewski fr st>]

Similar to this problem reported 3 years ago:
http://mail.gnome.org/archives/balsa-list/2001-January/msg00026.html

deleting a message should update the message window if that message is  
open.  Frankly, for navigation's sake, I think it should open the next  
message (see my recent message on the list) but even just closing the open  
window would be preferable to keeping it displayed.

Sorry I can't offer up a patch like the other guy.... :P

Thanks,
Andrew