Convert leaflet-editable-polyline to use polygons instead of polylines
Add safety code to prevent exceptions when `that._map` is null
For methods added to map, get map using `this` rather than `that._map`

Because `that._map` is null after the first polyline is removed from the map.
When a polyline is removed from the map, remove it from the internal polyline list.
Do display "new point marker" for first marker in list (since polygon is closed)
Distinguish between next/prev marker and next/prev point

* When getting next/prev marker, look in `_markers`
* When getting next/prev point, look in `latlngs()`