Step counter should not disappear when the execution is done

[feeley]

One possibility would be to display the step counter in the REPL itself, instead of the blank line. When execution is done the step counter would be added to the transcript.

For the novice user, the step counter may be confusing. So by default, the step counter should not be visible. One possibility is to only activate the step counter when the user does single steps the code (the play-pause button, or the play-animate button). When the play-infinite button is pressed there would be no step counter.

[dufour]

What if the play-infinite button is pressed during single-stepping or animation?

[dufour]

Fixed in 4151e419f6