_leaflet_id instead of _leafletId

removeWaypoint is referencing `_leafletId` as Marker/Layer ID, but Leaflet is using `_leaflet_id` (with underscore), see [Util.js (stable)](https://github.com/Leaflet/Leaflet/blob/stable/src/core/Util.js#L30).

This lead to getFirst returning null when the last waypoint has been deleted and the appending marker to be the previous of a deleted one instead of the last.