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

To avoid node->properties pointing to freed memory after each st_theme_unload_stylesheet, flush the cache that is node->properties. They will be reloaded as soon as they are required by ensure_properties.

And yes node->stylesheets_changed_id already existed, but was unused.

Why not just fix Croco? Croco does not use proper reference counting everywhere it should, and retrofitting it would be difficult due to the recursive nature of CRDeclaration.

Closes: #7339 ... but mostly reported in https://github.com/micheleg/dash-to-dock/issues/2179 and https://bugs.launchpad.net/bugs/2069559