If we want the model items to be listmodels, we don't need to do a ==
comparison. We need to do g_type_is_a(). Implementations of listmodels
are obviously fine.
The tree is not needed to walk around the nodes.
It is however still needed for anything that requires modifying the
tree.
There is no immediate benefit in changing this API, but there might be
situations in the future where we can avoid looking up the tree when we
just want to check some details about the node.
Variable, added, would be a garbage value if model is NULL and
the following code, if condition, use the uninitialized variable.
A side effect could be occurred by that.
To avoid, the variable is initialized to zero.
