[webcomponents] Replace attached/detached callbacks with insertedIntoDocument/removedFromDocument (#362)

Elements that define things or get used by other elements should probably do their work when they’re inserted into a document.  e.g. `HTMLBaseElement` needs to modify the base URL of a document when it gets inserted. To support this use case, we need callbacks when an element is inserted into a document/shadow-tree and removed from a document/shadow-tree.

Once we have added such callbacks, let us call them `insertedIntoDocument` and `removedFromDocument` callbacks, `attached` and `detached` callbacks seems rather arbitrary and unnecessary as the author can easily check the existence of the browsing context via `document.defaultView`.

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/webcomponents/issues/362