GimpOperationLayerMode instantiable and add a GimpLayerModeEffects
property to it that we check in ::process() to blend pixels
together.

* app/gegl/gimpoperationlayermode.[ch]: Do the change described
above. Currently only Normal and Addition are implemented. Normal
so that we don't need a special case for the gegl:normal op. Also,
the Dissolve layer mode is not a point op and needs to be
implemented elsewhere.

* app/gegl/gimpoperation*mode.c: Removed.

* app/gegl/gimp-gegl-utils.[ch]: Removed
gimp_layer_mode_to_gegl_operation().

* app/core/gimplayer.c

* app/gegl/Makefile.am
* app/gegl/gimp-gegl.c: Adapt.

svn path=/trunk/; revision=27400