handle relations in show mode

fix #306

porting the configuration for show templates from the orm admin.
getMappingType is never set it seems. @pulzarraider or is there something magic that phpstorm class usage did not find? or is this meant for manual overriding something?

also, the template name is hardcoded, analog to [orm admin](https://github.com/sonata-project/SonataDoctrineORMAdminBundle/blob/master/Builder/ShowBuilder.php#L126-L128). @pulzarraider should this not use getTemplate with the right type name?