from a different window:
* Made controller.notebooks responsible for preventing unmodified notes from being saved, instead
of model.Notebook handling this task.
* Created a revision validator for passing revisions as arguments to exposed methods.
* controller.Notebooks.save_note() now requires a previous_revision parameter, used to determine
whether the note has been modified in the particular window it's being saved from.
* save_note() returns a new previous_revision value, so the client can determine whether a save
has occurred from another window.
* controller.Notebooks.undelete_note() fixed to quietly bail if the note to undelete isn't
actually deleted, which can happen if it was undeleted in another window.
* Editor() now responsible for making revisions list if it doesn't exist
* No longer giving an "undo" message when the user deletes an empty note.
* On the client side, detecting whether the previous_revision as reported by save_note() looks
correct, and if not, alerting the user about the conflict. Also displaying a "compare versions"
button that opens both the current version and the previous version.
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