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/application.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 NSApplication;
+
+    unsafe impl ClassType for NSApplication {
+        #[inherits(NSObject)]
+        type Super = NSResponder;
+    }
+);

src/platform_impl/macos/appkit/mod.rs

@@ -0,0 +1,11 @@
+#![deny(unsafe_op_in_unsafe_fn)]
+
+mod application;
+mod responder;
+mod view;
+mod window;
+
+pub(crate) use self::application::NSApplication;
+pub(crate) use self::responder::NSResponder;
+pub(crate) use self::view::NSView;
+pub(crate) use self::window::NSWindow;

src/platform_impl/macos/appkit/responder.rs

@@ -0,0 +1,11 @@
+use objc2::foundation::NSObject;
+use objc2::{extern_class, ClassType};
+
+extern_class!(
+    #[derive(Debug, PartialEq, Eq, Hash)]
+    pub(crate) struct NSResponder;
+
+    unsafe impl ClassType for NSResponder {
+        type Super = NSObject;
+    }
+);

src/platform_impl/macos/appkit/view.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 NSView;
+
+    unsafe impl ClassType for NSView {
+        #[inherits(NSObject)]
+        type Super = NSResponder;
+    }
+);

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;
+    }
+);