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

If we have none, how about introducing `preventCapturing` property, (any better name is welcome), to an Event interface to control a capturing phase?

```webidl
dictionary EventInit {
  boolean bubbles = false;
  boolean preventCapturing = false;
  boolean cancelable = false;
};
```

I think our engines would get a benefit becuase it reduces a burden of an event dispatching.


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