Add a way to remember permission authorizations

How permission authorization work in QtWebEngine seems to differ from what e.g. Chromium (or also other browsers) do.

With other browsers, once permission authorization is granted for a page (can e.g. use this demo to test), the webpage is allowed to send notifications without having to re-ask for permissions every time, even after the browser is restarted.

With websites like e.g. Slack, that means the user has to click the "request permissions" banner and grant permissions every time, only then the permissions will work.

I can see two ways to fix that:

• Currently, the docs for QWebEnginePage::setFeaturePermission say Note: Call this method on the featurePermissionRequested() signal, as it is meant to serve pending feature requests only. Setting feature permissions ahead of a request has no effect.. If it was possible to call setFeaturePermission during initialization, this would allow setting all remembered permissions at the start.
• Or alternatively, QtWebEngine (or the underlying Chromium) could remember granted permissions by itself, though that would make it more difficult to manage/clear remembered permissions.