Add KmlExport for Place/Venue + refactoring of the

KML/KMZ rendering and creation as we now need to export not only Layer
documents but also Place and Vanue documents as well. As the Place and
Vanue documents have similar fields (primary category, description, links
etc.) those Placemark nodes of the KML can be rendered via a convenience
method in the now new BaseKmlRenderer. There's also only one ScpKmlExport
now that replaces the ScpLayerExport class and can be configured to use a
custom IKmlRenderer implementing renderer instance. This is used to
differentiate between Layer and Place rendering. The Venue export just
reuses the PlaceKmlRenderer.

There is now slightly "breaking" change as the category_map.json file has
now always a prefix of the module the categories are taken from. That is:

(layers|places|venues)-category-map.json
disable debug mode of imperia-scp exports