Added stretches layer, removed WW-Area named. "Waterway Area, named" is no longer needed as all ww-area is coming from our db, where different sources are merged. Instead we can now display stretches. Todo: stretches for different countries should be distinguishable (different colors)

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