Introduce the INodeTextProvider<T> (model plug-in) which extends
Comparable<?.

Implementing classes are Schema, DiagramLabel, SchemaArea, SchemaRecord
and Set - (with the exception of Schema, ) these are the classes that
have to provide a node text in the outline view.
Write debug messages when a listener is added to or removed from the
model change dispatcher.  Such a debug message clearly identifies the
listener and, when the listener is an edit part, prints out the direct
ancestors of that edit part.