In gimp_image_reorder_item(), call gimp_item_start/end_move()
before/after reordering the item (and use an undo group, so that
the resulting undo actions are grouped together with the reordering
undo action,) so that if the item is a child of a group layer, and
reordering moves it out of the group in a way that causes the
group's mask to be resized, the mask will be properly restored when
undoing the operation.