Basic .kmz support

Extends the KML plugin to handle .kmz files (zipped .kml files).
Introduces quazip as an optional dependency for that purpose: When
quazip is available, .kmz is available as well. Otherwise only .kml
support is available (as before).
TODO:
- Respect base url correctly (to load other resources from the archive)
- Register .kmz files to be handled by Marble (if quazip found)

DIGEST: Marble can now open .kmz files. See