dom: Add missing event handlers

Adding:
* global event handlers,
* window event handlers,
* document and element handlers,
* and support for BeforeUnloadEvent.

Signed-off-by: Piotr Stankiewicz <bionicrift@gmail.com>

components/script/dom/webidls/HTMLFrameSetElement.webidl

@@ -7,4 +7,5 @@ interface HTMLFrameSetElement : HTMLElement {
   //         attribute DOMString cols;
   //         attribute DOMString rows;
 };
-//HTMLFrameSetElement implements WindowEventHandlers;
+
+HTMLFrameSetElement implements WindowEventHandlers;