* app/widgets/gimpcontainerview.[ch]: add and remove container trees
  recursively. Change virtual function ::add_item() to pass the
  insert_data of the parent viewable.

* app/widgets/gimpcontainercombobox.c
* app/widgets/gimpcontainerentry.c
* app/widgets/gimpcontainergridview.c: changed accordingly.

* app/widgets/gimpcontainertreeview.c
* app/widgets/gimpitemtreeview.c
* app/widgets/gimplayertreeview.c: dito, but actually use the passed
  parent_insert_data to insert the item at the right place in the
  GtkTreeView.