[DMN Designer] Save Stunners MagnetConnector.auto property

Further to DROOLS-3702 we are approximating Stunner's MagnetConnector auto property when unmarshalling a DMN XML file.

Connectors made using the toolbox (either by adding a connector between two existing nodes or adding a new connected node) are auto=true. Moving a connector end-point to a Magnet manually (other than "centre") sets auto=false. It is these situations when re-opening the file can give a different visualisation of the connectors. We need to store the connectors auto property as a DMN extension if we are to ensure re-opening any file preserves the connectors appearance.