Remove existing layers on topic change

This PR reacts on topic changes in the catalog tree directive. Whenever the topic changes, all layers being part of the layer manager are removed from the map.

Please don't merge yet as I need feedback on the following:

1) Somehow I doubt that catalogtree is the right place for this. Wouldn't it make more sense to add it to the topic directive?
2) We are relying on a filter to get the layers to be removed. This has to be the same layer as the one used in the layermanager. We should centralize this filter. Where? (gaLayers is a candidate)