CAN note-existence handler

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• CAN note-existence handler should emit NOTE_EXISTENCE
  • So other systems can respond when a note is identified in CAN, the note-existence handler will fire a NOTE_EXISTENCE event with the following argument
• CANs note-existence handler should listen for SCRATCHFILE_ADDRESSED
  • So CAN can learn if the scratchfile contains a new note. The note-existence handler should listen to the SCRATCHFILE_ADDRESSED event. Once recieved, the handler should check if a note at this address exists in the notebook. CAN's note-existence handler should fire NOTE_EXISTENCE when it determines if a note exists.