API Change: Make GridFieldConfig objects decoratable

This change promotes GridFieldConfig objects to first-class Object descendants, making them extendable through the decorator pattern.

Relevant discussion in the dev group: https://groups.google.com/forum/#!topic/silverstripe-dev/Z6tu1k05ZmA
Merge pull request #2311 from unclecheese/patch-12

API Change: Make GridFieldConfig objects decoratable