Bring back the Panel!

A small but interesting PR, I guess it will be interesting to more than one people.
It is also a good way to observe the GC.

Many properties have their default values, and some modifiable properties are made constants so the user should not expect that changing the values is working (most notably `priorities.{high,medium}`).

@sjrd I copied the `*.oz` files from mozart1's repository. Is it the right way? I did not find any `lib/tools/panel/*` file in the git history.