re-design entityDescGet
* new interface SSEntityDescriberI for service spanning call getDescForEntity
* implementations handle filling the description objects themselves now (e.g. tags, discussions, user events, ...)
* implementations can add additional appropriate properties, but don't have to tide over common properties anymore; handled by abstract super class SSEntityDescA now
* added possibility to retrieve user events for given user and entity additionally within entityDescGet
* removed entity description types from SSEntityE