app: handle an index of -1 generically in GimpContainer::reorder()

by turning the -1 into n_children - 1 directly in gimp_container_reorder()
instead of having all subclasses deal with the -1 separately. Remove -1
handling from gimp_list_reorder().

Also, optimize reordering to the same index away without increasing
the function's cost (it was doing a gimp_container_have() before, now
it doees gimp_container_get_child_index(), which both have the same
cost).