ENH Add extension point for modifying readonly GridFields.

Developers may want to amend a gridfield (either by modifying its
components or amending the list) after it has been transformed to be
readonly.
One such modification may be replacing the GridFieldViewButton
with something else - because components are instantiated with the "new"
keyword this would be the only way to do that.

src/Forms/GridField/GridField.php

@@ -272,6 +272,8 @@ class GridField extends FormField
             $copyConfig->addComponent(new GridFieldViewButton);
         }
 
+        $copy->extend('afterPerformReadonlyTransformation', $this);
+
         return $copy;
     }