Add dialog closedby="" and requestClose()

The new closedby="" attribute for <dialog> can control what closes the dialog, including allowing new light dismiss behavior, or disabling close requests for modal dialogs.

The new dialogEl.requestClose() method acts as if a close request was issued by the user, firing the cancel event, and then (if the event isn't canceled) firing the close event and closing the dialog. This method does not require user activation. The existing closeWatcher.requestClose() method was updated to not require user activation either, to match this.

Closes whatwg#9373. Closes whatwg#10164. Closes whatwg#10592.

See also: openui/open-ui#834, openui/open-ui#950 (especially openui/open-ui#950 (reply in thread)), openui/open-ui#960, openui/open-ui#961.

Author: mfreed7