Clean up iOS class declaration (#2462)

* Begin abstraction over UIKit

* Clean up UIWindow override declaration

* Clean up UIApplication delegate declaration

* Clean up UIViewController override declaration

* Finalize objc -> objc2 rename

src/platform_impl/ios/uikit/view_controller.rs

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