Scoring Sheet Viewer: Refactor

[janezd]

What's wrong?

_class_combo_changed (https://github.com/biolab/orange3/blob/master/Orange/widgets/visualize/owscoringsheetviewer.py#L446C9-L446C29) checks whether the class indeed changed and if so, they (indirectly) call https://github.com/biolab/orange3/blob/master/Orange/widgets/visualize/owscoringsheetviewer.py#L459, which just negates some coefficients and subtracts risks from 100. I don't think this is very safe.

Switching back and forth can easily go wrong. The widget should remember the values for one target and use them to compute that values for the shown target. I suspect this may be the reason for failing test (see e.g. #6995).

How can we reproduce the problem?

Test fails randomly, but apparently only on github CI, not locally.