Fairness: more intuitive selection of performance and fairness metric in model comparison view

[romanlutz]

Long-term we want the dropdowns / selectors to be more intuitive in the model comparison view. Perhaps this could be handled similar to what the Explanationdashboard does, i.e. the selector is at the corresponding axis. This won't work for sensitive features, but for performance metric and fairness metric.

Given that they'll have quite a few entries we may want to make them searchable as well.

Related to #59