handle_scan_devices
Description:
Signal emitted when a remote caller is invoking the ScanDevices D-Bus method.
If a signal handler returns true, it means the signal handler will handle the invocation (e.g. take a reference to 
        invocation and eventually call 
        complete_scan_devices or e.g. return_error on it) and no
      other signal handlers will run. If no signal handler handles the invocation, the 
      UNKNOWN_METHOD error is returned.
Parameters:
| invocation | 
Returns:
| DBUS_METHOD_INVOCATION_HANDLED or  |