Introduce a single cross-platform KeyCode enum

We used to have a different `KeyCode` enum on every platform making it
unnecessarily hard to specify hotkeys in a cross platform manner. Here
we merge them all together into a single `KeyCode` enum. It is based on
the [UI Events KeyboardEvent code
Values](https://www.w3.org/TR/uievents-code/) web standard. This also
meant we now had to map the values from the individual operating systems
to these values. For this we use the tables on [MSDN - KeyboardEvent:
code
values](https://developer.mozilla.org/en-US/docs/Web/API/KeyboardEvent/code/code_values).
There are a few additional and alternative values, so the `KeyCode` enum
and its parsing got extended a bit beyond the standard to handle these.
The implementation for Windows and Linux also slightly changed. Both of
these used to use Keyboard layout specific hotkeys, which isn't what we
want. We may however want to introduce a way to resolve the hotkeys to
the current Keyboard layout for visualization purposes in the future.

crates/livesplit-hotkey/src/lib.rs

@@ -30,3 +30,30 @@ cfg_if::cfg_if! {
         pub use self::other::*;
     }
 }
+
+mod key_code;
+pub use self::key_code::*;
+
+#[cfg(test)]
+mod tests {
+    use std::{thread, time::Duration};
+
+    use super::*;
+
+    #[test]
+    fn test() {
+        let hook = Hook::new().unwrap();
+        hook.register(KeyCode::Numpad1, || println!("A")).unwrap();
+        println!("Press Numpad1");
+        thread::sleep(Duration::from_secs(5));
+        hook.unregister(KeyCode::Numpad1).unwrap();
+        hook.register(KeyCode::Numpad4, || println!("B")).unwrap();
+        println!("Press Numpad4");
+        thread::sleep(Duration::from_secs(5));
+        hook.unregister(KeyCode::Numpad4).unwrap();
+        hook.register(KeyCode::Numpad1, || println!("C")).unwrap();
+        println!("Press Numpad1");
+        thread::sleep(Duration::from_secs(5));
+        hook.unregister(KeyCode::Numpad1).unwrap();
+    }
+}