Allow tweaking of a cloned object prior to showing its admin form

This allows for twaks prior to diplaying the cloned object in its admin
form. For example, you could append " (duplicate)" to the object name.

NOTE: I added a sample use of the new feature in the sample project to advertise its use, but that had rather important consequences in the tests. Do you think it's too much? We could remove the sampleproject example, but then I don't know where we would document it. I'm a bit afraid to overcrowd the README (and we don't have full docs yet).
I'll check that tomorrow. So, you mean that the sample inline code I've added to the sample project wouldn't work if `do-not-clone` is present anywhere else than in the first comment? Interesting. I'll verify this and adjust the PR accordingly, thanks.