Bring parent items to the public API in the core

* app/core/gimpimage.[ch]: make the parent parameter public in
add_layer(), add_layers(), add_channel() and add_vectors().

* app/vectors/gimpvectors-import.[ch]: add parent parameters to
  the vectors import functions.

* app/core/gimpchannelundo.[ch]
* app/core/gimplayerundo.[ch]
* app/vectors/gimpvectorsundo.[ch]
* app/core/gimpimage-undo-push.[ch]: remember the parent item when
  removing layers, channels and vectors.

* app/actions/channels-commands.c
* app/actions/debug-commands.c
* app/actions/edit-commands.c
* app/actions/layers-commands.c
* app/actions/vectors-commands.c
* app/core/gimp-edit.c
* app/core/gimpimage-duplicate.c
* app/core/gimpimage-merge.c
* app/core/gimpimage-quick-mask.c
* app/core/gimplayer-floating-sel.c
* app/core/gimpselection.c
* app/core/gimptemplate.c
* app/dialogs/file-open-dialog.c
* app/display/gimpdisplayshell-dnd.c
* app/text/gimptext-compat.c
* app/tools/gimptexttool.c
* app/tools/gimpvectortool.c
* app/widgets/gimptoolbox-dnd.c
* app/xcf/xcf-load.c
* tools/pdbgen/pdb/image.pdb
* tools/pdbgen/pdb/paths.pdb
* tools/pdbgen/pdb/vectors.pdb: pass NULL as parent item to above
  functions and add FIXMEs all over the place because there is some
  more hacking needed to make adding with index = -1 (on top of the
  current item) work again.

* app/pdb/image-cmds.c
* app/pdb/paths-cmds.c
* app/pdb/vectors-cmds.c: regenerated.

* app/core/gimpimage-duplicate.c: duplicate the original image's
  tree structure in the copy.

* app/widgets/gimpitemtreeview.[ch]: add parent to GimpAddItemFunc,
  add utility function gimp_item_tree_view_get_drop_index() which
  figures where to add something dropped to an item tree.

* app/widgets/gimpchanneltreeview.c
* app/widgets/gimplayertreeview.c
* app/widgets/gimpvectorstreeview.c: changed accordingly, using above
  new GimpItemTreeView API.

app/dialogs/file-open-dialog.c

@@ -247,7 +247,8 @@ file_open_dialog_open_layers (GtkWidget           *open_dialog,
 
   if (new_layers)
     {
-      gimp_image_add_layers (image, new_layers, -1,
+      /* FIXME tree */
+      gimp_image_add_layers (image, new_layers, NULL, -1,
                              0, 0,
                              gimp_image_get_width (image),
                              gimp_image_get_height (image),