Implement vtkMRMLIGTLConnectorNode::Persistent

This option represents whether or not a connection should
be persistent across scene saves.

if `persistent == ::PERSISTENT_ON` then the node connects
based on the `::state` setting.

if `persistent == ::PERSISTENT_OFF` then the node does not
reconnect when the scene is loaded.
STYLE: Untabify + VTK-style indentation.