message flag change notifications

[pavlov netscape com (Stuart Parmenter)]

Since Messages and Mailboxes are now GtkObjects, I am going to make use of
their signal handling.  I've converted all the message flag change
notifications over and will probably check this in sometime after the
beginning of the year (it works now, but running it over a 28.8 modem to a
windows xserver kind of sucks and i want to make sure I don't break
anything...).  This should allow me to eliminate all of the MailboxWatcher
stuff and hopefully clean up the code some more.  Just wanting to let
everyone know.

Stuart