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/lib.rs

@@ -169,6 +169,22 @@ pub struct Window {
 #[derive(Debug, Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
 pub struct WindowId(platform::WindowId);
 
+/// Identifier of an input device.
+///
+/// Whenever you receive an event arising from a particular input device, this event contains a `DeviceId` which
+/// identifies its origin. Note that devices may be virtual (representing an on-screen cursor and keyboard focus) or
+/// physical. Virtual devices typically aggregate inputs from multiple physical devices.
+#[derive(Debug, Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub struct DeviceId(platform::DeviceId);
+
+/// Identifier for a specific analog axis on some device.
+#[derive(Debug, Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub struct AxisId(u32);
+
+/// Identifier for a specific button on some device.
+#[derive(Debug, Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub struct ButtonId(u32);
+
 /// Provides a way to retreive events from the windows that were registered to it.
 // TODO: document usage in multiple threads
 pub struct EventsLoop {