Allow users to rename "score" and "score2"

**Is your feature request related to a problem? Please describe.**
At present, user cannot define a sensible name for "score" (or score 2 in the multiobjective mode).

**Describe the solution you'd like**
It would be logical for the user to be able to rename the score to whatever they desire, and let this label flow down to the back-end to be applied to the plots as well. In a simple UI, it may make sense to let the default label be "Quality" in one-score mode, and "Quality" and "Cost" in two-score mode.