[webcomponents] Clean up: refactor event path calculation algorithm and retargeting algorithm (#124)

The event path calculation algorithm and retargeting algorithm should not have a strong dependency to *events*. In fact, these algorithms are re-used in `elementFromPath`, but it's confusing since events are not involved here.

Instead, these algorithms should be considered as more general algorithms, which are not related to events system. Define these in more general way, and make Events section (and other sections) should refer to it.

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/webcomponents/issues/124