Remove <applet>

* Removed applet from document.all[name] collection.
* Removed applet from document[name] collection.
* Removed applet from window[name] collection.
* The document.applets collection now returns an empty collection.
* Removed handling for applet in <iframe sandbox>.
* Removed rendering rules for applet.
* Removed the element itself and HTMLAppletElement.
* <applet> now uses HTMLUnknownElement.

The parser is intentionally not changed.

Fixes #454.