Use XInput2 for event handling

This provides smooth scrolling for touchpad devices and will
enable support for touch events etc. in future.

src/api/x11/mod.rs

@@ -7,6 +7,7 @@ pub use self::xdisplay::XConnection;
 pub mod ffi;
 
 mod events;
+mod input;
 mod monitor;
 mod window;
 mod xdisplay;