Allow introspection of WindowBuilder attributes

Makes WindowAttributes public and adds window_attributes() getter to
WindowBuilder.

In version 0.27, the WindowAttributes struct was made private, but this
removed the ability to introspect the default WindowBuilder values.

src/platform_impl/ios/view.rs

@@ -455,7 +455,7 @@ impl WinitUIWindow {
 
         this.setRootViewController(Some(view_controller));
 
-        match window_attributes.fullscreen {
+        match window_attributes.fullscreen.clone().map(Into::into) {
             Some(Fullscreen::Exclusive(ref video_mode)) => {
                 let monitor = video_mode.monitor();
                 let screen = monitor.ui_screen();