EBP doesn't save plan structure set

[cpinter]

When saving a scene, the "Structure set" and "Target volume" associated with a plan is not saved

Migrated from https://app.assembla.com/spaces/slicerrt/tickets/858-ebp-doesn--39-t-save-plan-structure-set/details

[cpinter]

Greg Sharp: The above is not strictly true. Structure set is restored if EBP module has previously been initialized prior to loading the scene. But is not set in UI when scene is loaded prior to initializing EBP module.