Extend service with searchCategories().

BREAKING CHANGES in Kiezatlas service and REST API:

- rename method and return type:
{{{
SearchResult searchGeoObjects(...) -> GroupedGeoObjects searchCategories(...)

GET /site/geoobject?search=...     -> GET /site/category/objects?search=...
}}}

- new method:
{{{
GeoObjects searchGeoObjects(String searchTerm, long clock)

GET /site/geoobject?search=...
}}}