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/BeforeUnloadEvent.webidl

@@ -0,0 +1,11 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+/*
+ * For more information on this interface please see
+ * https://html.spec.whatwg.org/multipage/#beforeunloadevent
+ */
+
+interface BeforeUnloadEvent : Event {
+  attribute DOMString returnValue;
+};