Function content_register_deserializer 

pub fn content_register_deserializer<T: 'static, P: Fn(&ContentDeserializer, &mut Option<T>) + 'static>(
    mime_type: &str,
    type_: Type,
    deserialize: P,
)

Registers a function to deserialize object of a given type.

Since 4.20, when looking up a deserializer to use, GTK will use the last registered deserializer for a given mime type, so applications can override the built-in deserializers.

mime_type

the mime type which the function can deserialize from

type_

the type of objects that the function creates

deserialize

the callback

notify

destroy notify for @data