Allow overriding of deletion widget in formsets

[Ties Jan Hefting]

Hi there!

In Django 3.0 `ordering_widget` and `get_ordering_widget()` were introduced (see #29956). The typical use case was to easily override the `ORDER` field in formsets that are updated in the frontend.

For the exact same reason, I'd be interested in an implementation of `deletion_widget` and `get_deletion_widget()`. Support for overriding the deletion widget has been briefly mentioned by @claudep in the pull request comments: https://github.com/django/django/pull/10660#issuecomment-440327124, but I couldn't find more discussions about it. I'm curious to hear what people in this mailing list think of the idea. Does it sound like a logical next step?

Thanks in advance,

Ties Jan