On the client-side, this signal is emitted whenever the D-Bus signal "SessionStateChanged" is received.
On the service-side, this signal can be used with e.g. emit_by_name to make the object emit the D-Bus signal.
| arg_old_session_state |
Argument. |
| arg_new_session_state |
Argument. |
| arg_session_state_failed_reason |
Argument. |