void on_row_double_clicked(object o, Gtk.RowActivatedArgs args)
{
TreeView tv = (TreeView) o;
TreeModel model;
TreeIter iter;
if (tv.Selection.GetSelected (out model, out iter)) {
//put selection in selected
selected = (string) model.GetValue (iter, 0);
//activate on_button_accept_clicked()
button_accept.Activate();
}
}