Editorial: major cleanup around firing and dispatching events

This aligns firing and dispatching of events with updated terminology in
the DOM Standard. Among the changes:

* A lot less usage of "default action" which isn't really a thing.
  Instead we make use of the return value of the fire and dispatch
  algorithms.
* Instead of saying things bubble or are cancelable we initialize the
  attributes as such.
* We no longer re-state defaults for isTrusted, bubbles, and cancelable,
  as that only leads to confusion when they are *not* re-stated.
* We now use the legacy target override flag rather than supplying a
  named argument.

This fixes #1713, but plenty of follow up issues remain:

* #805 for the remainder of "default action" usage
* #1394 for updating synthetic click events
* #1887 for removing "fire a simple event" usage
* #1893 for updating when checkboxes get checked
* #1900 for figuring out if event dispatch requires more hooks
* #1912 for revisiting isTrusted usage
* #1913 for updating synthetic mouse events
* #1922 for making more events composed

Author: annevk