Add export of KMZ files for "imperia-scp" export

In addition to the KML, JSON and Category index mapping file there will be
a KMZ file. That KMZ file has the KML file and additional image files
necessary for the Place placemarks of the KML file. The primary category
of a Place can contain an image that is used as an IconStyle for that
placemark in the KML file. Therefore the asset file itself has to be next to
the KML file and is thus put into a zipped KMZ container.

When enabling the 'debug' setting on the ScpLayerExport, the actual files
are created on publish and revoke, but no SSH connection will be used. The
PHP error log will contain some debug information about created temporary
files.

refs #18631