ListingBlock: Default column definition

... should be configurable thru plone.app.registry

![Screen shot 2013-03-12 at 5 03 25 PM](https://f.cloud.github.com/assets/437933/249480/7874ed5a-8b2e-11e2-9cda-af42f053c873.png)

Currently it's static. 
Use default_method