Still the same problem as ever with the Python binding where we have a
hard time creating GParamSpec, hence we make them from object
properties.
See: https://gitlab.gnome.org/GNOME/pygobject/-/issues/227#note_570031
But then again, the Python binding way to create GObject properties does
not seem to give us a way to use our custom param types (or I didn't
find how). So when I create a property with Gimp.RGB type in Python, it
doesn't appear as a GIMP_PARAM_SPEC_RGB to our C code, but as a
G_PARAM_SPEC_BOXED. So my trick is to check the value type instead.
Note that I check the default value, but in reality it doesn't seem to
work much either. Better than no support at all anyway.
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