Largely based on a patch by Ell, with the enum type renamed and
various small changes. Adds another axis of configurability to the
existing layer mode madness, and is WIP too.
from gimp_applicator_new() and gimp_gegl_mode_node_set_mode().
Compositing doesn't depend on the layer format any longer, only on the
layer mode. Painting with "use applicator" unchecked is still broken
in some cases and needs more fixing.
