* gegl/gegl-node.c: gegl_node_detect tied in with operations.
* gegl/gegl-operation.[ch]: Added new virtual method called detect
default implementation is to say it is us, if we've got defined data
at the position.
Changes in operation classes:
* gegl/gegl-operation-filter.c: just call our source op asking it.
* gegl/gegl-operation-composer.c: use aux if pixeldata is define there
(should be overriden in some subclass(es?))
* operations/core/shift.c: modify coordinates.
* operations/transform/affine.c: modify coordinates.
* bin/gegl-view.c: added (disabled by default) code to print which
node is detected upon mouse press in the view.

svn path=/trunk/; revision=1138