Implement data model using Codable

[tonyarnold]

This PR aims to decode and encode the configuration property lists using Swift's Codable protocol.

The PR is currently a work in progress, and is not ready for use.

let data = try Data(contentsOf: openedFileURL)
let decoder = PropertyListDecoder()
let configuration = try decoder.decode(Configuration.self, from: data)

What's left to do?

• Refine the model - this is a first pass, and some of the models (notably DeviceProperties and NVRAM) are basically glorified dictionaries right now.
• Decoding model to memory for use when the user opens a file
• Using the model to populate a proper document, as well as views
• Saving model back to disk
  • Saving should omit values that are equal to their defaults