QQuickWebEngineDownloadItem is not public

QQuickWebEngineDownloadItem is not public.  This appears to be an oversight, because the corresponding QWebEngineDownloadItem is public.  This requires programs like Angelfish to do some nasty hacks to use QQuickWebEngineDownloadItem:

https://invent.kde.org/network/angelfish/-/blob/master/lib/qquickwebenginedownloaditem.h?ref_type=heads

Which causes complexities for Debian trying to maintain security support for WebEngine in stable because Angelfish is accessing a private API.

https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1059164

https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1057755