iOS: Accessibility elements in native subviews ignored

In my application, a native WKWebView is added to the main UIView, similar to how QtWebView integrates WKWebView.

When enabling VoiceOver, with both my custom WKWebView and QtWebView, accessibility elements in the browser component (controls from the HTML like line edits and buttons) are ignored and cannot be properly selected.

This is because QUIView overrides the default accessibilityElements implementation with its own, but ignores subviews.

The attached patch fixes the issue for me, by including subviews in the reported `accessibilityElements`. It only fixes it for my custom WKWebView item though, not for my QtWebView test case, so that's why I haven't proposed it on Gerrit yet.