Revert "Remove _gtk_box_get_children"

Revert "Remove _gtk_box_get_children"

This reverts commit 572e9a04027e213082a5b257e5d662a5daa32667.

_gtk_box_get_children was not doing exactly the same than
gtk_container_get_children does, because the latter uses the forall
implementation of GtkBox that takes into account the children pack mode while
the former just iterated the list of children. This broke the order of
the buttons in a GtkButtonBox when they were packaged with PACK_END.