Enhance Monitor-StateVariables attribute displayed in UI.

Right now we display a dictionary: state_variable_range, plus an array variables_of_interest.
The dictionary is fine and user-readable, but the array represents zero-based indices in a private array of states (the keys in the dictionary). The private array is obviously never displayed in the UI, and the user will need to input in the array indices in that array.

We should fix this by adding a decent multi-selector for state variables supported by a model, and do a proper transformation from that select into the internal model usage.