* app/core/gimpitem.[ch]: Move the base GeglNode here in
preparation for moving the GimpLayer gegl:shift op to here as
well. After all, the offsets are properties of GimpItem and not
GimpLayer.

* app/core/gimpdrawable.[ch]: Delegate appropriate stuff to the
GimpItem base class, like part of the visibility toggling and the
creation and destruction of the GimpItem node.

* app/core/gimplayer.c
* app/core/gimpchannel.c
* app/core/gimpdrawablestack.c: Changed accordingly, use the new
function names.

* app/core/gimpitempropundo.c
* app/vectors/gimpvectors-warp.c
* app/vectors/gimpvectorsmodundo.c: Include gegl.h instead of
glib-object.h

svn path=/trunk/; revision=27521