New design ideas:
* One global instance, don't try to play nicely with other instances
  except via IPC.
* One process owns the terminal and the other processes communicate via
  IPC.
* Clear the whole terminal and use multiple lines.

What's implemented so far:
* Query the terminal for size.
* Register a SIGWINCH handler.
* Use a thread for redraws.

To be done:
* IPC
* Handling single threaded targets
* Porting to Windows
* More intelligent display of the progress tree rather than only using
  one line.