GridField doesn't have a valid readonly state if it's value is set to an Object
without `forTemplate()`. The default behaviour is to render a ReadonlyField,
but given GridField is a complex type this isn't suitable.
This bugfix provides a transformation method to render only components that are
whitelisted to provide a readonly state.
@see #3357 - https://github.com/silverstripe/silverstripe-framework/issues/3357
