Gridview and DataProviders: Autoconfig sorting

Gridview and DataProviders can now automatically recognize colums for sorting if they are not explicitly specified.

This gets a bit more convention over configuration feeling:

- auto fill sorting colums in ActiveDataProvider if none are configured
- Lazy fallback of label creation via Inflector for Sort class (do it only in link creation method to allow setting label by other classes for example GridView)
- Refactored DataColum to use model label or specified header for sorting link label