st: Only consider visible children for :first/:last-child

st: Only consider visible children for :first/:last-child

While mapping the :first/:last-child pseudo classes directly to the
ClutterActor:first-child/:last-child properties allows for an easy
implementation, it is unexpected that rules can appear to not have
an effect because the selected child is hidden. GTK's behavior of
applying the classes to visible children makes much more sense, so
change our implementation to do the same.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/312