LevelStreamingPersistence plugin improvements

UDN: [Link Removed]

• Fix SaveToFileCommand : [Image Removed]
• Problem with private properties serializing structs:
  [Image Removed]
• Add support for runtime instantiated UObject classes
• Add single restore callback:
  "Honestly I would question the value of the individualized callbacks, over a single restore callback that gets the list of properties, so in case you do care about what was restored, you have that context, but you aren't doing work with incomplete data. The other reason I made an interface, is because much of the post restore initialization I needed to do the lootable object class I was testing was on other properties unrelated to what was restored, and it doesn't seem practical that this restore logic would be externally located such as it is with the statically registered callbacks of ILevelStreamingPersistenceModule."

  class LEVELSTREAMINGPERSISTENCE_API ILevelStreamingPersistInterface
  {
      GENERATED_IINTERFACE_BODY()
  public:
      virtual bool ShouldPersistProperty(const FProperty* Property) const { return true; }
      virtual void PostRestorePersistedProperty(const FProperty* Property) {  }
      virtual void PostRestorePersistedProperties(const TArray<const FProperty*>& Properties) {  }
  }; 

• Support runtime (spawned) objects (see details in UDN)