[selection-api] Define the interaction with user-select CSS property (#46)

We need to define how selection behaves with respect to `user-select` CSS property.  We also need to ensure use cases listed at https://github.com/w3c/editing/issues/58#issuecomment-108860806 are satisfied.

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/selection-api/issues/46