Improved 'autoeditor - set' (aes) user experience.

Score Manager now provides warnings if composer attempts to set autoeditor for
a class not housed in existing output.py file.

Example:

    Red Example Score (2013) - materials - tempo inventory - select class:

       1: ClefInventory
       2: MarkupInventory
       3: NoteAndChordHairpinHandler
       4: NoteAndChordHairpinsHandler
       5: NoteRhythmMaker
       6: OctaveTranspositionMapping
       7: OctaveTranspositionMappingInventory
       8: PatternedArticulationsHandler
       9: PerformerInventory
      10: PitchRangeInventory
      11: ReiteratedArticulationHandler
      12: ReiteratedDynamicHandler
      13: RestRhythmMaker
      14: SargassoMeasureMaker
      15: TaleaRhythmMaker
      16: TempoInventory
      17: TerracedDynamicsHandler
      18: TimeSignatureInventory
      19: list

    > clef

    Existing output.py file contains TempoInventory.
    Overwrite existing output.py file?
    Ok?