This abstract spec type is basically a GParamSpecObject with a default
value. It will be used by various object spec with default values, such
as GimpParamSpecColor, GimpParamSpecUnit and all GimpParamSpecResource
subtypes. Also it has a duplicate() class method so that every spec type
can implement the proper way to duplicate itself.
This fixes the fact that in gimp_config_param_spec_duplicate(), all
unknown object spec types (because they are implemented in libgimp,
which is invisible to libgimpconfig) are just copied as
GParamSpecObject, hence losing default values and other parameters.
As a second enhancement, it also makes it easier to detect the object
spec types for which we have default value support in
gimp_config_reset_properties().
As a side fix, gimp_param_spec_color() now just always duplicates the
passed default color, making it hence much easier to avoid bugs when
reusing a GeglColor.
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