Preferences: allow formats other than .ini/ConfigObj

Currently `apptools.preferences` uses .ini files mediated by `ConfigObj`. This was a reasonable choice in 2008, but now there are other possible configuration file formats that might be reasonably used. Candidates include:

- JSON
- YAML
- TOML
- XML

Different applications may want to used different formats because they are easier to manually write, or because they need to be interpreted by other systems where .ini format does not have an easily available reader; or integrate with existing systems which have made different choices. Also the long-term future of `ConfigObj` is unclear - it works, but is also not being updated frequently.

Fortunately the actual file-handling code in the main `Preferences` is wholly contained within the `save` and `load` functions. It would be straightforward to add hooks to read from different formats based on file extensions; and to write to a particular format based on configuration of the preferences system.

One consideration is that the current system assumes that all data is text, while other systems permit additional data types for things like numbers, lists and mappings. This is primarily an issue for reading of files, particularly if they have been manually edited; other value types may need to be explicitly cast to strings.

Similarly, other formats have notions of hierarchical structure which have been imposed on the .ini format manually (by `.`-separated section names). Some thought may need to be put into how we want to handle this: do we want a flat structure with `.`-separated sections, or a nested structure using the components.