API CHANGE: Simplified state handling so that it's just a key store. Affectors are replaced with GridField_ActionProviders. API CHANGE: Removed GridField state manipulation actions instead opting for GridField_ActionProvider actions.
API CHANGE: Removed support for modifiers that add "body" rows, instead having core support for generating the body rows hardcoded into the GridField.
API CHANGE: Allow modification of columns across the whole GridField with the GridField_ColumnProvider interface.
API CHANGE: Renamed GridField_AlterAction to GridField_Action, and added actionName/args parameters, since it can be used for all actions (including batch actions and row actions)
API CHANGE: Removed GridFieldRow class.