Bring back the Panel!

A small but interesting PR, I guess it will interest more than one people.
It is also a good way to observe the GC.

Many properties have default values (`0` or `false`). Two mutable properties are made constant so the user should not expect that changing their values is working: `priorities.high` and `priorities.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.