Commit df7d8e2cb highlights a crash on test_destroy_destroy, in fact it could
happen that calling clutter_actor_destroy on a child while iterating on the
list, would implicitly call test_destroy_remove that tries to modify the list
at the same time. Causing a memory error.
So instead of manually free the children list, just ensure that this list is
valid and that when the object destruction is done, this is free'd.
See: https://gitlab.gnome.org/GNOME/mutter/merge_requests/576https://gitlab.gnome.org/GNOME/mutter/merge_requests/581
GList's used in legacy code were free'd using a g_list_foreach + g_list_free,
while we can just use g_list_free_full as per GLib 2.28.
So replace code where we were using this legacy codepath.
https://gitlab.gnome.org/GNOME/mutter/merge_requests/576
