Added "historyAdded" doc event.

Added a doc event "historyAdded" which is dispatched when a new entry is added to the document history. It is not dispatched when a history entry is amended. This allows CM's history to be integrated easily into application-level undo/redo.

For example, I have used this in an application that includes multiple CM instances with separate documents (not views on a single document, so linkedDoc does not help), and with additional external controls/data. I was able to listen for new history entries on the CM instances, and add corresponding entries into a unified application-level history stack.

Signed-off-by: Grant Skinner <info@gskinner.com>