Output shows no change between evaluation with No Results

[timothyhinrichs]

When the result of evaluation is "No Results", and the user runs 2 evaluations in a row, there is no way to know if the output has been updated or not because the output may be exactly the same across two evaluations. This is especially problematic when the evaluation takes several seconds or even a minute.

Ideas:

• Closing the output window at the start of a new evaluation.
• Wiping the output window at the start of a new evaluation.
• Including a timestamp or a duration on each evaluation, even when "No Results" is the result

[tsandall]

This is related to #10.