[selection-api] The spec doesn't define event handlers onselectstart and onselectionchange (#55)

The spec doesn't define any onfoo event handlers, but IIRC at least some implementations already have them.
Need to spec which interfaces should have onselectstart and onselectionchange. 

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/selection-api/issues/55