Removed RestRhythmMaker. MILESTONE.

Use Editor instead.

MILESTONE: all atomic editors have now been removed.

Only list editors remain:

    * Editor
        * ListEditor
            * ClefInventoryEditor
            * InstrumentationEditor
            * MarkupInventoryEditor
            * OctaveTranspositionMappingEditor
            * OctaveTranspositionMappingInventoryEditor
            * PerformerEditor
            * PitchRangeInventoryEditor
            * TempoInventoryEditor

List editors will be collapsed in coming commits.