allow specification of GridFieldAddNewMultiClass handler

I have the awkward need of overriding the doSave method of the GridField ItemRequest handler in order to provide some many_many_extrafields default values from the record being edited. And of course I really want to use the inline editor (which does not allow overriding the ItemRequest class as per GridFieldDetailView). Here's a patch that allows.

If interested I'm writing a gridfield based userforms-esque module. Would like to publish it when ready. Here's an example [gist](https://gist.github.com/briceburg/9a11858be7bc34fe128d) of what I'm trying to accomplish.

Thanks as always for your contributions!