* move checks to __construct so they can be run on $_POST data
and don't create malformed filter urls
* if we received malformed $_GET params, build new params and reload
* do not store error state in cache
* cleanup
* modernize DB-Types
- long term: should be split in class that describes the DB-Type and container class that handles multiples
* make unchanging filter props static, allow lookup of criteria indizes through filter
* move username/mail/password checks to util and make them usable as input filter
