* app/core/gimpitem.[ch]: Moved the shift_node from GimpLayer to
GimpItem, and call it offset_node. Also added
gimp_item_get_offset_node() meant to be used in the same way as
gimp_item_offsets().

(gimp_item_real_translate): Keep the offset_node up to date.

* app/core/gimplayer.[ch]: Don't manage the offset_node, get it
from GimpItem instead.

svn path=/trunk/; revision=27527