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/mod.rs

@@ -264,6 +264,7 @@ pub mod file;
 pub mod filelist;
 pub mod filereader;
 pub mod focusevent;
+pub mod forcetouchevent;
 pub mod formdata;
 pub mod htmlanchorelement;
 pub mod htmlappletelement;