[FEATURE] Configurable snapping distance

This adds the FRLayeredNavigationItem.snappingDistance property, which allows the 'snap' distance to be configurable. This is useful when there are effects underneath the layers to be displayed, or in conjunction with another feature (configurable content width vs. configurable layer width) PR which I'll be submitting shortly.

The default behavior is consistent with the current behavior of FRLayeredNavigationController.