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_traits/lib.rs

@@ -227,10 +227,23 @@ pub enum CompositorEvent {
     MouseMoveEvent(Option<Point2D<f32>>),
     /// A touch event was generated with a touch ID and location.
     TouchEvent(TouchEventType, TouchId, Point2D<f32>),
+    /// Touchpad pressure event
+    TouchpadPressureEvent(Point2D<f32>, f32, TouchpadPressurePhase),
     /// A key was pressed.
     KeyEvent(Key, KeyState, KeyModifiers),
 }
 
+/// Touchpad pressure phase for TouchpadPressureEvent.
+#[derive(Copy, Clone, HeapSizeOf, PartialEq, Deserialize, Serialize)]
+pub enum TouchpadPressurePhase {
+    /// Pressure before a regular click.
+    BeforeClick,
+    /// Pressure after a regular click.
+    AfterFirstClick,
+    /// Pressure after a "forceTouch" click
+    AfterSecondClick,
+}
+
 /// An opaque wrapper around script<->layout channels to avoid leaking message types into
 /// crates that don't need to know about them.
 pub struct OpaqueScriptLayoutChannel(pub (Box<Any + Send>, Box<Any + Send>));