This makes GTreeView able to support multi-column models!
Only one column can be the "tree column", this is column 0 by default
but can be changed by overriding GModel::tree_column().
This breaks GSortingProxyModel selection preservation across resorts.
I'm not yet sure how we're going to solve that, but it's going to have
to work a bit differently than before, since the model itself no longer
knows what's selected.
Selection is now managed by GModelSelection which allows us to select
any arbitrary number of items, and to have different selections in
different views onto the same model. Pretty sweet. :^)
This is just for convenience really. It lets you skip const_casting in
all the code that calls this API, which you would basically always be
doing otherwise.
