Use OpenLayers for maps

The commits replace the Google Maps V2 code with OpenLayers using Maps V3 (so no API key required anymore). For geocoding, OpenLS is used (since the Google geocoding API is somewhat unclear when not used with GMaps).

There are some open issues still, but I want to make sure that the overall design and implementation is acceptable.

TODO:
 - Avoid duplicate markers
 - Update markers on map change (move, resize)
 - maybe: split JS into separate file again
 - Update configuration management (no more google API key)
 - Provide support for alternative maps providers (eg. OSM), and UI to configure site defaults
 - Provide optional click&point UI in metadata editor (in addition to lat/long text fields)