Config Manager Cleanup

This is pull request contains the cleanup commits from pull request #371 minus the commit which removes operator[] from ConfigManager::Domain. This still allows nicer usage of Domain with [], which was only a problem for the notifications.

The basic idea behind these changes is that a Domain is not a generic StringMap thus subclassing feels wrong. This also allows use to introduce some neater features like preserveing the order of keys in our config file (not yet implemented) or Domaon::clear() to also clear the comments on the keys (not yet implemented). Thus, I consider this cleanup useful in any case (and digitall and fuzzie seemed to like it too).

In case nobody disagrees I will merge this tomorrow evening (12.8).