Rename and reduce ListVariableListener methods

Rename the methods according to the naming meeting agreement:

• listVariableChanged
• listVariableElementAssigned
• listVariableElementUnassigned

ListVariableListener will be slimmed down to:

• afterListVariableElementUnassigned(scoreDirector, element)
• beforeListVariableChanged(...)
• afterListVariableChanged(...)

afterListVariableElementUnassigned() is the only un/assignment method listeners need to implement. It is necessary so that listeners can unset shadow variables after an element is unassigned.

On the other hand, moves are required to go through the full ceremony and call before/afterElement(Un)Assigned. Although some of the calls are no-ops we require them anyway to avoid later finding out they're needed.