Re: [webcomponents] Need "slotting changed" event (#288)

Would the user facing api by like this?

```
new MutationObserver(function() {
  // react to slot changes...
}).observe(slotElement, {slotAssignment: true});
```

If so, this seems fine. Thanks!

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https://github.com/w3c/webcomponents/issues/288#issuecomment-176510066