Manage removal of Prefs that are unused

[windymilla]

If a pref is removed in a release, then when the user first runs the new release they may get a warning about an unknown pref.

Options:

1. Make the warning only visible for devs/debug or something similar (like assert), so it doesn't confuse regular users (maybe this or this )
2. Don't remove prefs, just make them unused, so they can still be read from a Prefs file without errors. Every so often, you could clear that list, once people have had a chance to pass run the release without the pref, so their prefs file won't contain the old pref any more.
3. Try this first: wrap assertion in try/except, and output the warning in the exception code.