[webcomponents] Add attributeFilter (#367)

At defineElement time,

1. Do a `Get(PassedConstructor, "attributeFilter")`.
1. If it is undefined,
  1. never queue attribute changed callbacks
  1. if attributeChangedCallback is present, throw a "what are you doing?" error
1. Run the Web IDL "convert to `sequence<DOMString>`" algorithm on it, rethrowing any errors
1. Store the resulting sequence as the attribute filter and use it to dispatch attributeChangedCallbacks.

There's no provision in v1 for "all attributes".

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/webcomponents/issues/367