Allow overriding part of a dictionary-type setting
|Reported by:||Claude Paroz||Owned by:||nobody|
|Cc:||Triage Stage:||Ready for checkin|
|Has patch:||yes||Needs documentation:||no|
|Needs tests:||no||Patch needs improvement:||no|
where it was desired that related settings could be grouped in a parent dictionary.
The idea is that the user shouldn't then mandatorily define all setting keys to only change some of them.