* app/core/gimpimage.[ch]: rename all gimp_image_reposition_foo() to
  gimp_image_reorder_foo() and added "new_parent" parameters. Factor
  out calculating of the item's new container and index to a utility
  function.

* app/core/core-enums.[ch]: rename the REPOSITION undos to REORDER.

* app/core/gimpimage-undo-push.[ch]
* app/core/gimpchannelpropundo.[ch]
* app/core/gimplayerpropundo.[ch]
* app/vectors/gimpvectorspropundo.[ch]: change accordingly. Remember
  the old parent item in all item reorder undos.

* app/widgets/gimpitemtreeview.h: change GimpReorderItemFunc prototype
  accordingly.

* app/widgets/gimpchanneltreeview.c
* app/widgets/gimplayertreeview.c
* app/widgets/gimpvectorstreeview.c (class_init): follow image API
  name changes.

* app/widgets/gimpitemtreeview.c (gimp_item_tree_view_drop_viewable):
  implement reordering of items between groups.

* app/widgets/gimpcontainertreeview.c
  (gimp_container_tree_view_reorder_item): fix to reorder the item
  within its level of the tree (unrelated to above changes, but needed
  to make them work).