The list of objects where the config object is a dedicated custom class (instead
of a runtime-registered class) is well known. These are the operations
registered inside gimp_operations_init().
The list inside gimp_gegl_procedure_execute_async() which the previous commit
was updating was not right: it was still missing "gimp:hue-saturation" and
"gimp:threshold" should not have been on the list (this was generating a
CRITICAL when trying to get the "config" property on this object).
Instead let's add some init/exit() functions in gimp-operation-config API to
distinguish the operations with custom config (registered during init) with all
the others. Then we add gimp_operation_config_is_custom() which can be used
everywhere where we want to verify if an operation is using a custom-made config
object or a generated class just mirroring the operation properties.
This way, we should not get out-of-sync anymore.
3f54d83bac