more callbacks and callback args for Show/EditCustomFields

* add AfterCustomFieldValue callback
* add "Grouping" and "ARGSRef" args to AfterCustomFields
* add "Grouping," "Object" and "ARGSRef" args to BeforeCustomFields
Add ProcessUpdates callback in Modify.html

Add a callback in Modify for running any update or processing
code. This is useful if you're adding some form fields and storing
values in attributes, for example.
Merge branch '4.2/custom-field-callbacks'