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.
The sort of the sortlistmodel is now stable with respect to the original
list model.
That means that if the sorter compares items as equal, the model
will make sure those items keep the order they were in in the original
model.
Or in other words: The model guarantees a total order based on the
item's position in the original model.
We need to keep this data around for changes in future commits where we
make the sorting stable.
An important part of the new data handling is that the unsorted list
needs to always be dealt with before the sorted list - upon creation we
rely on the unsorted iter and upon destruction, the sorted sequence
frees the entry leaving the unsorted sequence pointer invalid.
This change does not do any behavioral changes.
