appDisplay: Use the new org.gnome.desktop.app-folders schema for grouping

Rather than GMenu / app-folder-categories. This removes our last use of
gnome-menus in the stock gnome-shell, which is exciting, but also means
that app folders in Software start working.

Ideally, we'd have a button to launch our Software app as well from the
overview.

https://bugzilla.gnome.org/show_bug.cgi?id=722117