Add working, focus and idle/disconnected information in the progressbar

[triglian]

Currently the progress bar during an exercise only displays the number of people that have submitted. It would be great if we could also show more information for the set of people that haven't submitted yet. We should show a different color for the following categories:

1. working
2. focused (not idle)
3. idle/disconnected

The following graphic demonstrates this:

Don't forget to add tests :-)