User-defined Themes

[john-craig]

This pull request contains updates to the tnz library to make it possible for users to define themes for their ncurses environment and select them. This code was largely written by IBM Bob.

[najohnsn]

If I were asked, I would point to the last paragraph in https://ibm.github.io/tnz/colors:

If different colors are desired, check the zti-hosting terminal for the capability to change the color palette.
You may need to use TNZ_COLORS=8 to get zti to use the customized color palette.

The suggestion is to manage color themes through the hosting ascii terminal emulator and use the TNZ_COLORS=8 environment variable to get zti to use that theme.

Does this PR have an advantage of the above?

[john-craig]

Personally, I never had any luck with getting the terminal to honor colors through the TNZ_COLORS=8 method, so this approach makes that simpler. It also makes it possible for users to have a theme for their TNZ terminal different from their main terminal theme (if they so choose), and allows them to share their themes easily using the theme files.