RF: split BaseVisualStim into three classes

All test_all_visual/ tests are passing. New internal hierarchical structure:
- non-visual housingkeeping methods (MinimalStim) = the core of the core
- deprecated visual methods (LegacyBaseVisualStim)
- current visual methods (BaseVisualStim)

RatingScale benefits if it can inherit from MinimalStim, reducing code redundancy, look for another commit