[ENH] Improve scorer selection in Test and Score and predictions

[janezd]

Issue

• ScoreTable, which is used in Test and Score, and in Predictions, has a hard coded list of scorers, which defines the (default) visibility and the order. New scores are thus invisible by default.
• The names of classes are used for names of columns. It is thus impossible to translate them. A name attribute exists, but is not defined and defaults to class name.
• The user may not be aware that (s)he can choose additional scores by right-clicking the header.

Description of changes

• Scorers define default visibility and priority as class attributes.

• Visibility settings are stored as "hints", so new scorers (e.g. from add-ons) may be visible if they that's their default.

• The name attribute is now defined, but in settings the scorers are referred to by their class names, so settings "survive" the change of language.

• Add an indicator that can be clicked and opens a popup for selection of scores (fixes Test and Score - intuitive way to add new score #6131).

  Although currently in Orange.widgets.evaluate.utils, the class SelectableColumnsHeader is general and can be moved somewhere else and used, e.g. in OWRanks.

Includes

• Code changes
• Tests
• Documentation