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.
Better cleanup of area polygon.

Fixes OpenTreeMap/otm2-addons#38
Fixes #1265
Fixes #1264
Merge pull request #1269 from RickMohr/more-roof-fixes

Improve cleanup of area polygon