property-keys: gegl_param_spec_set_property_key; remove return type
gegl-op: add macro based alternative to varargs property meta-data

Varargs in operation initialization can cause crashes on startup; that might
be masked by running on 32bit vs 64bit; and possibly other platform specific
traits.
noise-rgb: declare property (metadata) with macros instead of varargs
color: use macros instead of varargs for prop metadata