Trait containing manually implemented methods of
Self::get_value but panics if the value is of a different type.
Gets the value of one or more cells in the row referenced by @iter.
The variable argument list should contain integer column numbers,
each column number followed by a place to store the value being
retrieved. The list is terminated by a -1. For example, to get a
value from column 0 with type
G_TYPE_STRING, you would
gtk_tree_model_get (model, iter, 0, &place_string_here, -1),
place_string_here is a #gchararray
to be filled with the string.
Returned values with type
G_TYPE_OBJECT have to be unreferenced,
values with type
G_TYPE_BOXED have to be freed.
Other values are passed by value.
a row in @self