Fixed issues related to scene opening and closing

There were multiple serious problems when a scene was loaded that contained saved OpenIGTLink  connections. These changes fix all the known issues.

Issues fixed:
* Nodes are not updating and duplicate nodes are created when a scene is loaded with an active OpenIGTLink connector (and only  duplicate nodes are updated by OpenIGTLink)
* Wrong model nodes are transformed and wrong models are shown/hidden when model nodes already exist in the scene and a scene is imported that contains OpenIGTLink connectors and locator models
* Socket connections remain open after scene is closed or connector node is deleted
* Display and pushOnConnect checkboxes are not working when Slicer is built in debug mode