forcetouch events support

This enables Apple forcetouch DOM events. It requires the preference dom.forcetouch.enabled.

The DOM events are described here:
- https://developer.apple.com/library/mac/documentation/AppleApplications/Conceptual/SafariJSProgTopics/RespondingtoForceTouchEventsfromJavaScript.html

The Cocoa mechanism is documented here:
- https://developer.apple.com/library/mac/documentation/Cocoa/Reference/ApplicationKit/Classes/NSEvent_Class/#//apple_ref/doc/uid/20000016-SW274

components/script/dom/webidls/ForceTouchEvent.webidl

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+/* -*- 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://developer.apple.com/library/mac/documentation/AppleApplications/Conceptual/SafariJSProgTopics/RespondingtoForceTouchEventsfromJavaScript.html
+
+/**
+ * Events: (copy/paste from apple.com)
+ *
+ *  webkitmouseforcewillbegin: This event occurs immediately before the mousedown event. It allows you to
+ *   prevent the default system behavior, such as displaying a dictionary window when force clicking on a
+ *   word, in order to perform a custom action instead. To prevent the default system behavior, call the
+ *   preventDefault() method on the event.
+ *  webkitmouseforcedown: This event occurs after the mousedown event, once enough force has been applied
+ *   to register as a force click. The user receives haptic feedback representing the force click when this
+ *   event occurs.
+ *  webkitmouseforceup: This event occurs after a webkitmouseforcedown event, once enough force has been
+ *   released to exit the force click operation. The user receives haptic feedback representing the exit
+ *   from force click when this event occurs.
+ *  webkitmouseforcechanged: This event occurs whenever a change in trackpad force is detected between the
+ *   mousedown and mouseup events.
+ *
+ */
+
+
+[Pref="dom.forcetouch.enabled"]
+interface ForceTouchEvent : UIEvent {
+    // Represents the amount of force required to perform a regular click.
+    readonly attribute float SERVO_FORCE_AT_MOUSE_DOWN;
+    // Represents the force required to perform a force click.
+    readonly attribute float SERVO_FORCE_AT_FORCE_MOUSE_DOWN;
+    // force level
+    readonly attribute float servoForce;
+};