[RFC] User-only settings

We discussed this at a meeting and postponed the idea, but I've thought about this a bit more...

There's a use case for saving a Setting locally, as opposed to in the workflow file.
- recent paths (OWFile) and any absolute paths (widgets inheriting OWSaveBase) (biolab/orange3#4919)
- vim mode (OWPythonScript) (biolab/orange3#4402)

This could be done not with QSettings, but the following way:
- each workflow is assigned a randomly generated ID
- each widget is assigned a sequential ID
- a JSON file named `.json` is created somewhere in the data directory (~/Library/Application Support/Orange on Mac), mapping widgets to their user-only setting values

The issue names OWFile, OWSaveBase, OWPythonScript, QSettings, and the data directory, but no implementation files or tests. Start by locating those widget classes and their setting handling, then clarify the workflow/widget ID and JSON storage design; done requires an agreed, tested implementation.

Written by the indexing model from the issue text.