Introduce the concept of hidden steps

In XWiki various job progress step are automatically generated in some potentially expensive area as a way to track time spent on them (queries, etc.). The main target being the performance tree enabled using debug=true in the URL.

But it sometimes makes a bit more complex to understand why the progress suddenly jump a step when you did not had any step around those database queries for example.

An easy way to avoid this would be to introduce "hidden" steps: steps not taken into account in the progress calculation but visible in the performances tree.