Finish up commit 17583ff0, which
ported GimpGradient from ::duplicate() to ::copy(), by doing the
same for the rest of the GimpData subclasses that implement
::duplicate().

We still keep GimpData's ::duplicate() virtual function around,
even though it now points to the default implementation (which uses
::copy()) for all subclasses, since ::copy() is stronger than
::duplicate(), and we might want to have certain GimpData types
that are duplicatable, but not copyable.