workspacesView: Update visibility after updating workspaces

When the number of workspaces changes, it is possible that one
of the visible (non-active) workspaces is removed, and another
invisible workspace moves in its place.

Make sure that workspaces are shown as necessary in this case.

https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/6519

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2699>
(cherry picked from commit 95386a14)