Correct erroneous g_object_unref calls

commit e89158e8abd2dae27d985e3574eb5aa1d265fc66 placed g_object_unref calls after every instance
of *_set_model - In most instances this was correct, but not all.  When (for example)
gtk_tree_view_set_model (view, model) is called, it takes a ref to model, but it will then
unref it upon the next call to _set_model and/or when view is destroyed.
Unreffing the model ourself, is therefore appropriate only when we have created it ourselves.
This change fixes the few instances where this was not the case.