Simplify all view model APIs and always return G_TYPE_OBJECT as the
item-type for every model.
It turns out nobody uses item-type anyway.
So instead of adding lots of APIs, forcing people to think about it and
trying to figure out how to handle filter or map models that modify item
types, just having an easy life is a better approach.
All the models need to be able to deal with any type of object going
through anyway.
Add an api to retrieve the model containing a given
item in a flatten listmodel. This is useful when the
individual items in the list don't have backpointers.
The code previously forgot to include the left child of the model's
node. Which of course only happened if that child wasn't NULL, which is
a common case.
Found and test provided by Matthias Clasen.
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.
