st/theme-node: Forget properties cache on theme changes

Croco does not use proper reference counting everywhere it should, and retrofitting it would be difficult due to the recursive nature of CRDeclaration. So to avoid node->properties pointing to freed memory after each st_theme_unload_stylesheet, flush the cache that is node->properties immediately upon stylesheet change. They will be reloaded as soon as they are required again by ensure_properties.

Closes: #7339