1. Modified gridfield to use default colours
2. Added ability to show and hide the filter
3. Design changes to icons used
4. Gridfield template fixes: Took out broken mark-up from templates
(spans were being inserted inside spans), and adjusted styles. Added
extra class to template
5. Added "filter by ..." placeholder text to input fields
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.