Support dealing with certificate errors in Qt WebView

The Qt WebEngine module allows one to handle certificate errors via the `certificateError` signal of the `WebEngineView` type and accepted/reject the error. This is very useful to allow displaying a web page despite a self-signed certificate is used.

It would be great if Qt WebView would allow the same on platforms that allow this like Android.

Note that it would also be ok to add a simple flag to allow insecure HTTPs connections in general. This is of course less ideal because it had to be used with great care. However, it would be better than nothing. A different way to cut corners might be adding a user visible confirmation page similar to what normal web browsers have.