Store the content of the editor on a per-level basis

[Tim Martin]

When the user goes to a level they have never seen before, the editor could be initialised with the content of the editor on the previous level. But after that, they should be able to edit the code without losing the code from the previous level.

This may be academic, since there isn't currently any way to navigate to an earlier level. However, it's annoying during testing if nothing else.