[webcomponents] Spec the Slots proposal (#289)

Let me use this *meta* bug to spec the Slots proposal.

I'm feeling that I need to update not-a-few places in the spec. I will take a note about the incoming spec changes here.

I might also propose a new API, and spec it, as an editor's draft, because some of the existing APIs wouldn't match the new model. e.g. `destinationInsertionPoints`.

See also the following issues:
- Related issues: #92, #128, #129 
- For Blink: #130.


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Reply to this email directly or view it on GitHub:
https://github.com/w3c/webcomponents/issues/289