onUnknownModifier

Handle a request to create an unknown layout modifier tag.

This function will be invoked every time a modifier with an unknown tag is used. For example, three widgets each using the unknown modifier will see this function invoked three times. Modifiers are serialized as a chain, so updating an unrelated modifier in a chain which also contains an unknown modifier will cause this function to be invoked again. Use the tag to de-duplicate the callbacks if desired.