no default application type; accept only valid application type
bugfix: variable name
bugfix: nested arrays must be merged recursive
partly rewrite as config is a nested array now (not flattened anymore)
restored the methods 'flatten' and 'deflatten' because those methods may still be useful when working with collections
cleanup
don't add prefix to key
changed key prefixing
deflatten input array and use array_replace_recursive to merge defaults with configuration
key-prefix must be added when storing new config value