WIXFEAT:4149 - Add support for variables and radio buttons in thmutil, and update WixStdBA to use the new functionality.

The title says it all.

Next steps:
1. Continue adding functionality to thmutil so WixStdBA doesn't have so many special controls.
2. Design and implement the Content idea to make it easier to have one control whose text changes depending on context.
3. Update the built-in themes.