As a consequence, here are needed new functions:
* libgimp:
- gimp_param_spec_resource_defaults_to_context()
- gimp_param_spec_resource_none_allowed()
* libgimpbase:
- gimp_param_spec_file_get_action()
- gimp_param_spec_file_none_allowed()
- gimp_param_spec_unit_percent_allowed()
- gimp_param_spec_unit_pixel_allowed()