Clean up macOS class declaration (#2458)

* Begin abstraction over AppKit

* Clean up NSApplication delegate declaration

* Clean up NSApplication override declaration

* Clean up NSWindow delegate declaration

* Clean up NSWindow override declaration

* Clean up NSView delegate declaration

src/platform_impl/macos/appkit/window.rs

@@ -0,0 +1,14 @@
+use objc2::foundation::NSObject;
+use objc2::{extern_class, ClassType};
+
+use super::NSResponder;
+
+extern_class!(
+    #[derive(Debug, PartialEq, Eq, Hash)]
+    pub(crate) struct NSWindow;
+
+    unsafe impl ClassType for NSWindow {
+        #[inherits(NSObject)]
+        type Super = NSResponder;
+    }
+);