Sets the function used to map items.
The function will be called whenever an item needs to be mapped and must return the item to use for the given input item.
Note that `GtkMapListModel` may call this function multiple times on the same item, because it may delete items it doesn't need anymore.
GTK makes no effort to ensure that map_func conforms to the item type of this. It assumes
that the caller knows what they are doing and the map function returns items of the appropriate type.
| this |
a `GtkMapListModel` |
| map_func |
map function |
| user_data |
user data passed to |
| user_destroy |
destroy notifier for |