Structure to be included in the class structure of objects that use this mixin.
Initialize it with tp_presence_mixin_class_init.
If the protocol imposes a limit on the length of status messages, one should implement get_maximum_status_message_length. If
this callback is not implemented, it is assumed that there is no limit. The callback function should be set after calling
tp_presence_mixin_class_init, like so:
TpPresenceMixinClass *mixin_class;
tp_presence_mixin_class_init ((GObjectClass *) klass,
G_STRUCT_OFFSET (SomeObjectClass, presence_mixin));
mixin_class = TP_PRESENCE_MIXIN_CLASS (klass);
mixin_class->get_maximum_status_message_length =
some_object_get_maximum_status_message_length;All other fields should be considered read-only.