Add callback method to certains functions

[LeMyst]

Find a way to add some callbacks method to allow control over part on the add/update/remove process.

These callback will be Empty/Optional by default, but wbi_helpers will provide helpful/default methods (like the "is_good_refs" one)

As a user I want to have access to a callback function for certain method to allow manipulating the data before updating because I don't want to iterate over all the existing data before importing new ones.

[dpriskorn]

What is the use case for this? I'm missing a good user story. As a user I want to ... because ...