Editorial: add "add an event listener" hook

This hook ensures that any special casing in addEventListener() is shared with event handlers. This commit also makes numerous editorial improvements that were long overdue around dictionary members.

The corresponding change to the HTML Standard, which defines event handlers, is tracked by whatwg/html#3561.

This helps with #365 and w3c/ServiceWorker#1004. (Tests will be added as part of #365 eventually.)