Add setting method to set image animation policy

Chromium has animation_policy in its WebPreferences which can be used to control how GIF images are animated (allowed, only once without looping, not animated at all).

With Chromium, this can be controlled using an Animation Policy extension which uses the chrome.accessibilityFeatures.animationPolicy API. No command-line switch seems to exist, so this can't be set with QtWebEngine at all as far as I'm aware.

Similarly to QTBUG-84486, this probably only requires a new method plus enum in QWebEngineSettings.