Return folder names as unicode subclasses

[menno]

Currently methods that return lists of folder name objects return (flags, delim, name) triples. It might be more convenient for the caller to return a unicode object with flags and delimiter attributes that can be accessed if required. Doing this could also remove the need for the folder_encode property.

The alt-folder-names repo experiments with this.

[menno]

Less keen on this due to the API change. Perhaps as a different call.