* app/widgets/gimpcontainertreeview.[ch]: Don't expose class
instance struct members that is currently only used within the
GimpContainerTreeView implementation.

* app/widgets/gimpcontainertreeview-private.h: New file containing
the definition of the private struct.

* app/widgets/gimpcontainertreeview-dnd.c: Change accordingly.

* app/widgets/Makefile.am: Add new file.

svn path=/trunk/; revision=27662