Fixes #2944. Makes ColorScheme explicitly readonly, removes Colors.Base, etc..., Fixes intermittent TreeView unit test failures

[tig]

Fixes

Third (or fourth?) try is a charm???

• Fixes TreeView sometimes causing unit test failure #2944

Proposed Changes/Todos

• Debug ColorScheme as it seems the likely culprit.
• Moved ColorScheme and Color to ColorSchemes.cs
• Beefed up unit tests
• Fixed likely TreeView test but: FakeDriver was not setting PlatformColor to -1 in Make
• Made Color and ColorScheme more robust by
  • Making ColorScheme effectively immutable. Changing it to readonly struct (value type) will require too many code changes at this point... I'd rather wait for Colors: ColorSchemes refactor - Should honor terminal color settings etc... #2381 to do that.
  • Removing old Colors.Base, etc... All code now uses Colors.ColorSchemes["Base"] style.
• Updated API docs

Pull Request checklist:

• I've named my PR in the form of "Fixes #issue. Terse description."
• My code follows the style guidelines of Terminal.Gui - if you use Visual Studio, hit CTRL-K-D to automatically reformat your files before committing.
• My code follows the Terminal.Gui library design guidelines
• I ran dotnet test before commit
• I have made corresponding changes to the API documentation (using /// style comments)
• My changes generate no new warnings
• I have checked my code and corrected any poor grammar or misspellings
• I conducted basic QA to assure all features are working