This change removes the default postback handler for the datatable
widget (onSave) and instead relies on the closest form's request
handler. While `onSave` was a safe default, it did not take into account
the relation widget using other handlers.
The table widget will use an explicitly specified handler first, then it
will try and use the form's request handler, before using `onSave` as a
fallback, although this should really never occur.
Fixes#4776.
2f500ab034