[whatwg/fullscreen] Dispatch fullscreenchange on the fullscreen element (#122)

This proposal has rough implementer agreement:

* `fullscreenchange` should dispatch on the element that goes fullscreen and have bubbles and composed set to true. That way it's still compatible with `Document`.
* We should move `onfullscreenchange` to the `GlobalEventHandlers` mixin (using a partial mixin I suppose).
* `fullscreenerror` and `onfullscreenerror` remain as-is, which is a little awkward.

This will help when there are a lot of shadow roots that each need to observe when the document goes fullscreen. Rather than having a single target with loads of listener that all need to run each time, we'd end up with lots of targets with few listeners.

Background: https://github.com/w3c/webcomponents/issues/614.

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