Unsubscribe one Action, rather than all actions for an Event Type

Add an option to Unsubscribe one Action subscribed to an Event Type,
instead of clearing all registrations for that Event Type.  This allows
multiple parts of the program to listen to the same event and respond
differently, and unsubscribe selectively (e.g. when a UI control is
destroyed).
removed unused using statement
added unit test CanUnregisterSingleSubscriptions
updated version number