Fix password prompt for Gtk2

Minor fixes to make the password prompt work with GTK2:

* GTK2 requires IDs on all objects, added some dummy values.
* `new_from_string` isn't available, `new().add_from_string` is equivalent.

The tray is however completely broken in GTK2, as `popup` isn't exposed via introspection, and would need to be re-implemented in PyGTK. Not worth the effort imo.