Separate backup

Recommended to review by commit as I try to structure it as refactoring goes. My intention is also to use it as tutoring material for some refactoring presentation.
I'm not sure if I prefer the old ```backup_device``` name or the new one. I understand that the sense of action is already in the class name, so it sounded redundant. But maybe the problem is that class names should not sound like actions.

I would go for something like ```Bootloader::Backup::store_pr_for(device)```

But you know, naming...