Refactor and initial tests for backuptool.

### Motivation
Bringing side modules up to scratch. Most of this was actually done a long time ago.

### Changelog
* Various little code and documentation changes; see the commit message for details.
* Separation of code from tests.

### Note
This is not urgent at all, but nicely showcases some of the python idioms. Have a look.
If you do, and an `onSuccessCallback` is specified, it will run. This doesn't sound logical to me. Like "if I don't tell you what to do in case of failure, pretend you just succeeded".