Expose separate track color in theme

[bszollosinagy]

Make the choice of color for the Slider tracker, and the unused portion of the Progress Bar and Meter more explicit. It now becomes a design choice in the theme, rather than being hard coded.

Motivation:

1. The original Bootswatch.com Litera theme has darker Slider trackers, which means that it can be a reasonable design choice
2. The light themes have a Slider tracker that is hard to see, same with the progress bars. In some designs this is perfectly fine, but in others it makes the design hard to parse visually.
3. Let the designer of the theme decide... don't link this parameter in a hard coded way

I have updated all the standard themes to expose the originally hard coded color, to this is expected to be a drop-in replacement.

Before this modification:

• For light themes:
  • the tracker color is always colors.light (except when the widget is specifically styled "light", then it's colors.bg)
• For dark themes:
  • the tracker color is always -20% Value (HSV) adjusted from colors.selectbg

After this modification:

• Exposed to themes with the color name "track", and initialized to the color scheme that used to be hard-coded, to allow a drop-in replacement.

Tested by running TTKCreator and trying all themes: python -m ttkcreator

[bszollosinagy]

Original Color Scheme for Scale:

Possible new one (or any other, see the "track" color spec the lower left corner)