Add WebAuth support for SAML-based authentication

In some deployments, users are required to use hardware security devices to authenticate themselves e.g. YubiKey.

QtWebEngine has now added support for WebAuth handling. When a page requests WebAuth authentication, the request is passed to the host application through a signal. The application must then handle the request by, for example, displaying a dialog to the user.

The QtWebEngine project implements a reference host application: simplebrowser, which implements such a dialog. This patch imports this code, makes various style improvements and connects it to the QWebView.

Signed-off-by: Joel Holdsworth joel@airwebreathe.org.uk