Persistent settings are used in followup PRs #15936 to unify gui settings
between bitcoin-qt and bitcoind, and #15937 to add a load_on_startup flag to
the loadwallet RPC and maintain a dynamic list of wallets that should be loaded
on startup that also can be shared between bitcoind and bitcoin-qt.
Writes the GUI settings to `guisettings.bak` in the data directory
before wiping them. This can be used to retroactively troubleshoot
issues (e.g. #11262) where `-resetguisettings` solves the problem.
Russell Yanofsky