On Windows and MacOS, add Window::has_focus

src/platform_impl/ios/uikit/window.rs

@@ -28,5 +28,8 @@ extern_methods!(
 
         #[sel(makeKeyAndVisible)]
         pub fn makeKeyAndVisible(&self);
+
+        #[sel(isKeyWindow)]
+        pub fn isKeyWindow(&self) -> bool;
     }
 );

src/platform_impl/ios/window.rs

@@ -348,6 +348,10 @@ impl Inner {
         None
     }
 
+    pub fn has_focus(&self) -> bool {
+        self.window.isKeyWindow()
+    }
+
     #[inline]
     pub fn set_theme(&self, _theme: Option<Theme>) {
         warn!("`Window::set_theme` is ignored on iOS");