VariableListener becomes public API

Decided:

• Rename clearWorkingSolution(ScoreDirector) to close().
• Should VariableListener have resetWorkingSolution() and close()?
  • Yes, but it must both be default methods.
• Can VariableListenerAdapter be removed?
  • Yes, those methods can be made default
• Should StatefulVariableListener be public API?
  • No, now that VariableListener gets those methods.
  • Should we rename it?
    • SourcedVariableListener
  • Should the VariableListener javadoc mention it?
    • No, purely internal.
• Should VariableListener implement Supply?
  • No, if the following is yes:
    • Can VariableListener not implement Supply, but SourcedVariableListener do implement Supply?
      • Probably: because custom VariableListeners are never demanded as a supply and all internal onces should be SourcedVariableListener.

Delayed:

• Should the VariableListener methods be defaulted? Yes, the before* methods should be, can do later.
• Should Supply exist as an interface? Maybe.