Revisit scrollbar colours and general styling structure

[rock3r]

Some thoughts:

• Scrollbar colours on Win/Linux appear slightly different than the Swing versions, even though our styling seem to be correctly set up. We should look into it, but it's not super important as it's a very small difference, difficult to notice
• The current structure is a bit convoluted, with macOS and Windows/Linux both sharing ScrollbarVisibilities even tho on Win/Linux you really can't change that, and the scrollbars across OSes behaving and looking fairly differently.
  • We could consider creating separate implementations for Mac, and Win/Linux, with separate styling if need be. This would make things easier in some places, but harder in others.
  • We most likely want to separate both the tab strip and the editor styles (see Implement thin and Editor-style scrollbars #559 and Improve scrollbars in tabstrips #553) as distinct ScrollbarVisibility types, too