client: make layer factory only return new layer config for individual maps instead of each time it is invoked. The purpose of the factory was to support multiple maps with individual layers. But returning a new config each time it is invoked leads to bugs that rely on the layer's state. Now this factory reuses the same objects it created before, per map.

teichmann@intevation.de = sascha.teichmann@intevation.de
bernhard.reiter@intevation.de = bernhard@intevation.de