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.