add key_code to get key's virtual code on keydown/up events

https://developer.apple.com/documentation/appkit/nsevent/1534513-keycode?language=objc

src/appkit/event/mod.rs

@@ -6,7 +6,7 @@ use objc::runtime::Object;
 use objc::{class, msg_send, msg_send_id, sel};
 
 use crate::events::EventType;
-use crate::foundation::{id, nil, NSInteger, NSPoint, NSString};
+use crate::foundation::{id, nil, NSInteger, NSPoint, NSString, NSUInt16};
 
 /// An EventMask describes the type of event.
 #[bitmask(u64)]
@@ -83,6 +83,14 @@ impl Event {
         characters.to_string()
     }
 
+    /// The virtual code for the key associated with the event
+    pub fn key_code(&self) -> NSUInt16 {
+        // @TODO: Check here if key event, invalid otherwise.
+        // @TODO: Figure out if we can just return &str here, since the Objective-C side
+        // should... make it work, I think.
+        unsafe { msg_send![&*self.0, keyCode] }
+    }
+
     /// The indices of the currently pressed mouse buttons.
     pub fn pressed_mouse_buttons() -> NSUInteger {
         unsafe { msg_send![class!(NSEvent), pressedMouseButtons] }

src/foundation/mod.rs

@@ -87,4 +87,6 @@ pub type NSInteger = libc::c_long;
 #[cfg(target_pointer_width = "64")]
 pub type NSUInteger = libc::c_ulong;
 
+pub type NSUInt16 = libc::c_ushort;
+
 pub type NSPoint = core_graphics::geometry::CGPoint;