Gio::ListModel::get_object(): Don't try to cast to Glib::Object
Gio::ListModel::get_object(): Don't try to cast to Glib::Object

Gio::ListModel::get_object() returns Glib::RefPtr<Glib::ObjectBase>.
Don't try to dynamic_cast it to Glib::Object. It would fail if the object
has been constructed as an interface, and e.g. has a Gio::File wrapper.
Fixes #93

GNOME/glibmm,GNOME/glibmm,GNOME/glibmm,GNOME/glibmm,GNOME/glibmm