message: ported userdefinedstruct api to 4.15
diff: #include "Slate/UEPySlate.h"
  +#if ENGINE_MINOR_VERSION < 18
  +#define USoftObjectProperty UAssetObjectProperty
  +#define USoftClassProperty UAssetClassProperty
  +typedef FAssetPtr FSoftObjectPtr;
  +#endif
  +
  DEFINE_LOG_CATEGORY(LogPython);
  