Auto merge of #9529 - paulrouget:focusEvents, r=asajeffrey

Implement focus, blur, focusin and focusout events

Based on https://github.com/servo/servo/pull/7985

Fixes https://github.com/servo/servo/issues/7981

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components/script/dom/webidls/FocusEvent.webidl

@@ -0,0 +1,14 @@
+/* -*- Mode: IDL; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* 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/. */
+
+// https://w3c.github.io/uievents/#interface-FocusEvent
+[Constructor(DOMString typeArg, optional FocusEventInit focusEventInitDict)]
+interface FocusEvent : UIEvent {
+  readonly attribute EventTarget?   relatedTarget;
+};
+
+dictionary FocusEventInit : UIEventInit {
+    EventTarget? relatedTarget = null;
+};