In 99.9% of all cases, these are just NULL, NULL.
So just do away with these arguments, people can
use the setters for the rare cases where they want
the scrolled window to use a different adjustment.
There is no agreement that a coverflow widget is
appropriate for GTK 4.
It would be ok as a demo if it could live in gtk-demo,
but that requires us to make GtkListBase public first.
The demo is also somewhat rough and needs more work
to look plausible.
Drop GtkCoverFlow and the related demo for now.
