st: Delay handling of :first/:last-child changes

st: Delay handling of :first/:last-child changes

Updating the :first/:last-child pseudo classes can result in a lot
of unnecessary style changes when bulk-adding children to a container,
as every child ends up as the new last child.

Address this by deferring the style change to an idle, so we only do
the work once for the actual first and last child.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/529