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

Okay, despite of what I've said earlier, I don't think we can implement this feature without making everything slower even in WebKit.  There are many optimizations I have to remove from the current implementation, and there will be a lot more work if we wanted to fire `slotchange` event precisely when the assigned nodes or distributed nodes change.

The problem isn't so much that assigned nodes / distributed nodes.  Just figuring out whether a slot content has changed or not is expensive if we had to do it precisely.  At this point, I'm inclined to say that this feature should be punted to v2, or indefinitely because I'm not convinced that browsers can provide a more efficient implementation than authors just calling `assignedNodes({flatten: true})` on every `requestAnimationFrame` to check whether the slot content has changed or not.

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