The workspace.js file is quite large and is a bit confusing when it
comes to the term "window" in there, because it can either refer to a
WindowPreview of a complete window or to an individual window like an
attached dialog.
So try to avoid that confusion and split the new WindowPreview class and
its WindowPreviewLayout layout manager out into a new windowPreview.js
file.
https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1307
