Use gimp_image_get_layer,channel,vectors_list() instead of
gimp_image_get_layer,channel,vectors_iter(). As a side-effect,
simplified some code that was making sure we don't modify the list
returned by the old functions (the new functions return newly
allocated lists so we can safely remove items from the image while
iterating it). Some places will need adjusting once we really have
item trees.