Add export and import for non-default registry settings

[pbauer]

I'm not 100% sure this is a good idea since importing settings from oder to newer versions only makes sense when checking every single setting manually. Also a lot of settings in Plone 4 are still in portal_properties and not covered here. The default Plone upgrade takes care of migrating these into the registry these but with exportimport this obviously does not happen.

On the other hand the ability to only export settings that were changed from the default can certainly make migrating much easier.