Feature: maintain order of sections and settings within file

[fluffynuts]

Currently, in-memory storage for INIFile is performed via .net generic dictionaries, which do not guarantee order when enumerating. The feature to rename sections has highlighted the volatility of order. It would be nice to (perhaps optionally? perhaps always?) retain ordering, which would require some custom work -- possibly an internal implementation if IDictionary<TKey, TValue> since OrderedDictionary isn't generic.