This (doesn't quite) fix an obscure part of the tool preset bugs: when
changing the brush on a GimpPaintOptions instance, we might or might
not change the brush size, aspect etc. to the set brush's native
values, possibly overwriting the intended values from a tool preset.

Implement GimpConfig::copy() and copy the affected values again after
the entire object has been copied, so we actually use the value of
gimp_config_copy()'s "source" object.

This would fix that particular tool preset problem if there wasn't
another bug that is still unfixed, stay tuned...