Add a demo mod for the GUI override feature

Mods can now ship replacements of our python files. We should create one just to demonstrate the feature and make it as a template for others. Most easily it would be just another component of gemrb-tweaks.

Start with the gemrb-tweaks components and inspect how mods currently replace Python files through the GUI override feature. Add a small demo mod there that visibly exercises the override and can serve as a template for other mods. Done means the component is included with gemrb-tweaks and clearly demonstrates the replacement behavior.

Written by the indexing model from the issue text.