Map background needs POI labels

[marians]

For better orientation, we should provide labels of well-known public places.

This is the current look of an area north-west of Heumarkt:

This is the same area in the OSM.org style:

While I think that OSM.org shows too many POIs for our purpose, I guess that some buildings like Gürzenich, Wallraf-Richartz-Museum and others should be labeled.