[Grace-core] fixing REPL's

[Marco Servetto]

Indeed, the old *user interface* for that kind of incremental
programming comes from a time where a decent GUI allowing editing was
not possible.
what we need is a good code visualization technique, where the student can see
big chunk of (now) not relevant code compacted in a single line with
the usual [+] to the side, and a good play button in the top.