Throw an exception if a Move or a Problem Change calls beforeVariableChanged() on a list variable

Ideally, make it impossible by making beforeVariableChanged accept BasicVariableDescriptor. This might require redesign of the variable descriptors type hierarchy. BasicVariableDescriptor is now a child of GenuineVariableDescriptor. Without changing that, it wouldn't be possible for variable listeners to change "basic" shadow variables.

The proposal above only applies to the InnerScoreDirector, that takes a variable descriptor as one of the arguments.

The public APIs (ScoreDirector, ProblemChangeDirector, IncrementalScoreCalculator) identify a variable using the variableName string. In this case it's only possible to detect the illegal call in runtime and throw an exception.