message: Add comment explaining explicit empty constructor
diff: NAS2D::Point<int> MOUSE_COORDS; /**< Mouse Coordinates. Used by other states/wrapers. */
  +// Explicit constructor needed in implementation file
  +// If a default constructor was used instead, construction would happen in importing translation unit
  +// The header uses forward declares for some types, so only incomplete types are available to importing code
  +// Complete types are needed to construct the std::unique_ptr members
  GameState::GameState()
  {}
  