This class encapsulates events which occur on BlueJ classes.
The following events can occur:
STATE_CHANGED - the compile state changed (either from uncompiled to compiled,
or from compiled to uncompiled)
CHANGED_NAME - the class has changed name.
REMOVED - the class has been removed.
In the case of STATE_CHANGED there are three possible sitations:
isClassCompiled() returns true: the class has been compiled successfully.