Support back/forward cache (bfcache)

From https://web.dev/articles/bfcache:

Back/forward cache (or bfcache) is a browser optimization that enables instant back and forward navigation. It significantly improves the browsing experience, especially for users with slower networks or devices.

When going to the devtools > Applications > Back/forward cache in Chromium, and clicking the button on e.g. qt.io, it shows:

Doing the same in simplebrowser shows:

From a quick Chromium code search, most implementations of WebContentsDelegate override IsBackForwardCacheSupported to always return true, but the base implementation returns false, and WebContentsDelegateQt does not override that.

It's unclear to me whether anything else other than returning true there is required for proper support, however.