Simplifies the design because derived classes can have whatever save UI class
they need and implement the void(void) set_ui_item_from option and
set_option_from_ui_item with whatever functions are appropriate for
the UI class. No need for callbacks or std::function members.
Separating the UI from the data model. Note that the GncOptionVariant
classes still have a GncOptionUIType member to ensure that a
GncOptionUIItem of the right type is attached.
