Expose certificate chain and cipher of currently viewed URL

The 5.14 release brought QWebEngineCertificateError::certificateChain(), but such information would be useful not just in case of the errors but also for properly loaded pages, so application could expose information about certificate to the user.

Related information are cipher used by this connection and mixed content state (which could be obtained using page specific URL interceptor but it's error prone and time consuming).

Both are available in QtWebKit, thanks to QtNetwork APIs.

Instead of simply exposing several new getters on the QWebEnginePage  it could be encapsulated as something like QWebEnginePageInfo or something like that.

It could include other properties in the future too, like actual text encoding used by the page, MIME type, document size etc.