Advanced progress manipulation, allowing to construct a single progress indication bar from multiple, possibly independent sources.

Note: The APIs in the package resemble the APIs from the main package but are separate.
There are the same visualization options as with the regular progress indication, with additional customization options provided.

- There are multiple, possibly independent sources of task's progress.
- These progress sources (contributors) can be added even during the task's run.
- The infrastructure assures that the progressbar never steps back but recalculates the remaining space among the running contributors.
- It's possible to monitor the progress from a central place.