[webcomponents] Remove `Callback` suffix from custom elements callbacks (#414)

As discussed during Jan F2F and the issue #362, we should not have "Callback" suffix at the end of each lifecycle callback.

Authors and frameworks shouldn't be overloading the meaning/semantics of those callbacks for other purposes (since this is a new feature in the first place) and authors prefer shorter names according to https://github.com/w3c/webcomponents/issues/362#issuecomment-179729388.

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/webcomponents/issues/414