addLayer - works like setLayers, but doesnt unselect all other layers so that selectFeature still will be working for the old layers
This depends on pull request: https://github.com/tomcollins/openlayers/commit/0a50181df09a2687d6a35305bd6d1de10f7541f8 otherwise you get problem described by:https://github.com/openlayers/openlayers/issues/958