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/forcetouchevent.rs

@@ -0,0 +1,59 @@
+/* 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/. */
+
+use dom::bindings::codegen::Bindings::ForceTouchEventBinding;
+use dom::bindings::codegen::Bindings::ForceTouchEventBinding::ForceTouchEventMethods;
+use dom::bindings::codegen::Bindings::UIEventBinding::UIEventMethods;
+use dom::bindings::global::GlobalRef;
+use dom::bindings::inheritance::Castable;
+use dom::bindings::js::{Root};
+use dom::bindings::num::Finite;
+use dom::bindings::reflector::reflect_dom_object;
+use dom::uievent::UIEvent;
+use dom::window::Window;
+use util::str::DOMString;
+
+#[dom_struct]
+pub struct ForceTouchEvent {
+    uievent: UIEvent,
+    force: f32,
+}
+
+impl ForceTouchEvent {
+    fn new_inherited(force: f32) -> ForceTouchEvent {
+        ForceTouchEvent {
+            uievent: UIEvent::new_inherited(),
+            force: force,
+        }
+    }
+
+    pub fn new(window: &Window,
+               type_: DOMString,
+               force: f32) -> Root<ForceTouchEvent> {
+        let event = box ForceTouchEvent::new_inherited(force);
+        let ev = reflect_dom_object(event, GlobalRef::Window(window), ForceTouchEventBinding::Wrap);
+        ev.upcast::<UIEvent>().InitUIEvent(type_, true, true, Some(window), 0);
+        ev
+    }
+}
+
+impl<'a> ForceTouchEventMethods for &'a ForceTouchEvent {
+
+    fn ServoForce(&self) -> Finite<f32> {
+        Finite::wrap(self.force)
+    }
+
+    fn SERVO_FORCE_AT_MOUSE_DOWN(&self) -> Finite<f32> {
+        Finite::wrap(1.0)
+    }
+
+    fn SERVO_FORCE_AT_FORCE_MOUSE_DOWN(&self) -> Finite<f32> {
+        Finite::wrap(2.0)
+    }
+
+    // https://dom.spec.whatwg.org/#dom-event-istrusted
+    fn IsTrusted(&self) -> bool {
+        self.uievent.IsTrusted()
+    }
+}