listview: Change how binding is done

listview: Change how binding is done

We now don't let the functions create widgets for the item from the
listmodel, instead we hand out a GtkListItem for them to add a widget
to.

GtkListItems are created in advance and can only be filled in by the
binding code by gtk_container_add()ing a widget.
However, they are GObjects, so they can provide properties that the
binding code can make use of - either via notify signals or GBinding.