When we remove a child, we stop its transitions (animations), but we
didn't stop animations on grand children. What we did, however, was to
clear the stage views of the grand children, and this caused a bunch of
orphaned transitions (ClutterTimeline) and accompanied warnings.
Make it so that if we stop transitions, and clear stage views, also stop
transitions for the grand children. Detached children don't have a way
to continue animating anyway, since they have no stage view (thus frame
clock) to be driven by.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2716>
a897542876