GtkListBox: fix model binding refcount issue

GtkListBox: fix model binding refcount issue

As it is, GtkListBox model binding will work nicely as long as your
create_widget_func returns a floating reference on the newly-created
widget.

If you try to return a full reference (as any higher-level language
would do) then you will leak that reference.

Fix that up by converting any floating references into full references
and then unconditionally releasing the full reference after adding to
the box.

https://bugzilla.gnome.org/show_bug.cgi?id=746893