Question bank plugins currently implement actions as a subclass of columns. They are returned by plugin_feature::get_question_columns then separated from the actual columns to be used in the action list on each row. This is due to the way they were historically displayed as a separate column for each icon, but makes no sense with the changes since 4.0.

This issue will re-factor the question bank actions API so that actions are their own class, separate from columns. Shared code will be inherited from a common base class to avoid duplication.