Remove clipboardchange event (and replace it possibly with a promise returning method)

[smaug----]

The current definition for clipboardchange event is such that it doesn't work too well with the model Safari and Firefox have (paste requires explicit click on a button).
And if we require always transient user activation before the event, it would be more ergonomic to have something like
element.onclick = () => clipboard.checkCurrentClipboardTypes().then((listOfBuiltinTypes) => { /do something/});

(I'm not saying Firefox would necessarily implement that API, since it is still rather privacy sensitive)