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

For developers, it will be good to have an event that is fired whenever slotting algorithm runs within a shadow tree. Maybe runs _and_ produces changes compared to the previous result? Don't know. Also don't know if this is v1 or v2.

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/webcomponents/issues/288