[FEATURE] Persisted information

[sdatkinson]

Implement something like this gist, but use JSON for data storage.

Can put state of messages to be shown once (e.g. #425), default model & IR path defaults (#315), default, presets (#252)...

But so long as this can ensure a file's existence at e.g. /Users/steve/Library/Application Support/NeuralAmpModeler/settings.json (modulo home directory) with setup pending to read & write, this can be called good.