Richer input events

This expands input events to represent sub-pixel mouse positions, devices responsible for generating events, and raw
device-oriented events. The X11 back end is refactored to make full use of the new expressiveness. Other backends have
had new functionality minimally stubbed out, save for the macos backend which already supports sub-pixel mouse
positions.

src/api_transition.rs

@@ -57,6 +57,9 @@ macro_rules! gen_api_transition {
         #[derive(Debug, Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
         pub struct WindowId(usize);
 
+        #[derive(Debug, Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
+        pub struct DeviceId;
+
         pub struct Window2 {
             pub window: ::std::sync::Arc<Window>,
             events_loop: ::std::sync::Weak<EventsLoop>,