Dispatch touch events and perform default touch actions.

This is currently limited to simple single-touch actions. It does not include
momentum scrolling or pinch zooming.

components/msg/compositor_msg.rs

@@ -160,5 +160,12 @@ pub enum ScriptToCompositorMsg {
     GetClientWindow(IpcSender<(Size2D<u32>, Point2D<i32>)>),
     MoveTo(Point2D<i32>),
     ResizeTo(Size2D<u32>),
+    TouchEventProcessed(EventResult),
     Exit,
 }
+
+#[derive(Deserialize, Serialize)]
+pub enum EventResult {
+    DefaultAllowed,
+    DefaultPrevented,
+}