It's not behaving as expected here. For instance, Ultimaker 3 wasn't specifying has_machine_materials and thus only the base materials would get loaded, but clearly the Ultimaker 3 has materials specialised for it.
Whether or not a printer has materials specialised for it is now determined by whether the specialisations exist in the material files. So we don't need the metadata entry any more. It seemed to have not been in use anyway, except by one printer which specified that has_machine_materials is true. I've now made it behave as if it's always true.
Contributes to issue CURA-6831.
Among the machines, variants, materials, qualities and intents, only machines and materials can ever be added during runtime. For the rest, we don't need to listen to these signals.
Contributes to issue CURA-6600.
