* app/xcf/xcf-private.h: add properties PROP_GROUP_ITEM and
  PROP_ITEM_PATH

* app/xcf/xcf-save.c: when saving a group layer, save a
  PROP_GROUP_ITEM.  When saving a child item, save a PROP_ITEM_PATH
  which contains the path indices returned by gimp_item_get_path().

* app/xcf/xcf-load.c: when loading a PROP_GROUP_ITEM, replace the
  layer that is being loaded by a GimpGroupLayer, also ignore that
  layer's hierarchy (it makes no sense to load the tiles of a layer
  that's generated from its children). When loading a PROP_ITEM_PATH,
  pass the loaded path up to xcf_load_image() so it can add the loaded
  layer at the right place in the tree.