[filechooser] Unref cancellables we got from the model, not the ones that come into...

[filechooser] Unref cancellables we got from the model, not the ones that come into the callback

The model_cancellables have an extra ref when we get them from gtk_tree_model_get().
So, we need to unref them always.  On the other hand, the cancellables that get
passed as arguments to the callbacks in question are memory-managed by
GtkFileSystemModel.

Reported by Morten Welinder <mortenw@gnome.org>

https://bugzilla.gnome.org/show_bug.cgi?id=646460