The listener is notified only for the specified type of
configuration changes. When the listener is interested in more than
one event type this method has to be called multiple times.
Alternatively it can register as universal listener that will be
called for all event types. However, this option is provided
primarily to support debugging and monitoring.
Parameter xListener
The new listener.
Parameter sEventType
The event type that the listener is interested in. The set of
event types is not fixed and there can be no exhaustive
list. The empty string is a special value in that the listener
will be called for all types of event.
Parameter aUserData
Arbitrary data that is passed to the listener when it is called
for the specified event type. When one listener is regisered
for more than one event type then different user data objects
may be given as well. Supplying unique integer values allows
the listener to use a switch statement to distinguish between
the different event types.
