Qt WebView: Add API to disable user interaction

There are some native ways to disable interaction in the Android/iOS WebView, e.g.:

• https://stackoverflow.com/questions/3853794/disable-webview-touch-events-in-android
• https://stackoverflow.com/questions/53092140/wkwebview-mac-os-disabling-user-interaction

It would be handy to have a cross-platform QML API for this, like enabled: false. Or perhaps control could be more fine-grained (e.g. can scroll but can't select anything)