Re: [w3c/webcomponents] Consider adding a childrenChangedCallback or something (#619)

If there is such an implementation, I think it should be called both on when the children are first added as well as when children change.

There's a [similar proposal](https://github.com/skatejs/skatejs/issues/863) and I've been playing around with similar implementations in the corresponding PR. The problem arose from needing to get nodes for a certain slot so that we could `render()` out information based on those nodes, such as a TodoMVC implementation where you need to show the todo count based on the number of items (in this case, added as light DOM).

It seems that if a `childrenChangedCallback()` existed, that we'd be able to optimise this implementation. Currently, the PR for that issue has to completely subvert `slotchange` and use a `MutationObserver` because that event doesn't fire when the children are first attached to the host, but only on subsequent updates. This is problematic for reasons mentioned in the issue.

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