We currently avoid previews from overflowing in most setups by artificially limiting them to a maximum of six workspaces. Add some proper handling to also cover cases where space is more limited, and to allow removing the restriction in the future. For that, wrap the previews in an auto-scrolling scroll view and add overflow indicators on each side. Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/merge_requests/316>