Re: [webcomponents] Introduce "delegatesFocus" flag to shadow root (#126)

The `focusable` or `setForceFocusable` were introduced to the document trying to explain some of the
native elements via shadow DOM.

For example `<a name=...>` isn't focusable but `<a href=...>` is focusable, without having to add/remove `tabindex` attribute in HTML.  So it could be used for emulating such behavior, but I don't think it is a viable use case, and we can omit it at least for v1.

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