[clipboard-apis] Wording around default actions needs updating (#14)

The model, as discussed in [https://www.w3.org/Bugs/Public/show_bug.cgi?id=12230](bug 12230), is:
```JavaScript
if (target.dispatchEvent(event))
  doSomethingByDefault();
```

In other words, instead of saying "the default action of the beforecopy event is to ..." and (in one place) "If the event's default action is not prevented", the spec should consistently look like the cut action, where it's "fire a clipboard event named cut" followed by "if the event was not cancelled".

Also s/cancelled/canceled/ everywhere :)

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/clipboard-apis/issues/14