Change the way folder modification events are signalled. Now there

Change the way folder modification events are signalled.  Now there
are several different types of events, and each type has specific
parameters associated with it.  The intent of this change is to allow
the front end to figure out what is happening in the back end and
reflect that to the user.

Also: eliminate MESSAGE-MODIFICATION-COUNT and eliminate a handful of
modification events that were not visible at the folder abstraction
boundary (events are now part of the abstraction boundary, and should
not reflect irrelevant internal state).