* bin/gegl-node-editor.[ch]: (gegl_node_editor_class_init),
(property_editor_general), (gegl_node_editor_class_set_pattern),
(gegl_node_editor_class_matches), (gegl_type_subtypes),
(gegl_type_heirs), (gegl_node_editor_new): Added some more logic
towards easier extensibility of custom property editors.
* bin/gegl-node-editor-brightness-contrast.c: specify op.
* bin/gegl-node-editor-level.c: specify pattern "opacity:threshold".